Not so long ago, when he was a lance corporal in the Argyll and Sutherland Highlanders, he'd train by marching for miles through landscape like this while carrying 25kg of kit. He used to say the only way he would stop was if he dropped down dead, and that relentlessness remains. Barclay just keeps walking and talking, never stopping to glance behind him at Loch Long, the village of Arrochar or the train speeding towards Oban. Yet in another sense, all this man does is look back.

"I'd rather have had an arm or a leg blown off," he says. "At least people could see that injury. When it comes to mental injuries, they can't. But there's nothing worse than losing your mind."

Barclay, who lives in Dalmuir, has post-traumatic stress disorder (PTSD), a psychological condition caused by his experiences of combat. He is one of thousands of ex-servicemen and women in the UK who suffer from it, and numbers are increasing all the time as a result of the conflicts in Iraq and Afghanistan, and the unique pressures these have put on the military. The threat of roadside bombs and suicide bombers mean soldiers feel under threat constantly. Experts believe it's more challenging psychologically than traditional battles, in which opposing forces lined up against each other. There is also the added pressure of the army being undermanned and overstretched, leading to less time off between deployments than is healthy.

It can often take considerable time after an individual leaves the services for the symptoms of PTSD – including nightmares, flashbacks and uncontrollable rage – to manifest, and years before the sufferer decides to seek professional help. But Combat Stress, a charity specialising in the treatment of combat-related psychiatric illnesses, is already treating a stream of veterans of the current Iraq conflict, and it is thought that significant numbers of traumatised Afghanistan vets will be attending the charity's residential centres by 2012. In the past three years, these have seen a 53% increase in new referrals. There is talk about a mental health time-bomb ticking towards detonation.

Hollybush House, run by Combat Stress, is Scotland's only residential rehabilitation centre for ex-servicemen and women with PTSD. It's a grand 19th-century house in the South Ayrshire countryside. As part of the treatment programme, the team runs an annual adventure training weekend, taking clients to the Argyll and Sutherland Highlanders cadet centre for two days of biking, canoeing and other activities, including climbing the Cobbler. The idea is that men – the veterans are mostly male, with the very occasional exception – who were once incredibly physical, but who have been living too long in their own heads, will benefit from strenuous challenges. Last year, an ex-soldier who had barely left his flat in 18 years reached the summit and burst into tears, so great was his sense of achievement.

Barclay, by contrast, is still pretty fit, having only quit the army at the end of 2005. He joined at the age of 25 and saw some rough scenes in Northern Ireland before being deployed to Iraq in 2003, as part of the build-up of forces in Kuwait which then crossed the border. His two tours each lasted six months, during which time he survived a suicide bomber blowing himself up in a car as he passed in a convoy.

PTSD is a condition so thoroughly unnerving and debilitating that the bland clinical term itself seems a pale joke. During the First World War they referred to shell-shock, but for as long as there has been war, even going back to the ancient Greeks, this condition has been recognised. These days it's thought to be caused by a specific event, and for Barclay, in common with a lot of the men interviewed for this article, it wasn't the actual fighting that he found traumatising.

A breakdown in communications meant he was sent to war without a rifle, ammunition or body armour. Worse, he had no NBC (nuclear, biological, chemical) suit and no canister for his gas mask. It was two weeks before Barclay received his kit, but by that time it was too late. He felt totally vulnerable and had lost faith in the system. He was anxious, panicky and troubled by morbid thoughts. "It got to the stage where I'd be lying in my billet at night just hoping a mortar would land on me."

We're trudging up a well-worn path through purple heather. Barclay's shaved head and freckled face is covered in a sheen of sweat, but from exertion not anxiety. We could be a couple of weekend Munro-baggers treating ourselves to a Corbett, and it feels unreal to be talking about bad dreams and desert deaths. Yet for Barclay, Iraq is more real and present than the Scotland to which he has returned. Only four days ago he was in a psychiatric ward, unable to cope with the nightmares that sour his sleep and the intrusive thoughts that give his waking hours the coppery tang of blood.

Though he'd been feeling better for a while, he got up one morning in late June and switched on the news only to learn that his friend, Lance Corporal Jimmy Johnson, had been killed in Afghanistan. Barclay has lost a lot of pals and feels so guilty about still being alive that he keeps his medals out of sight in a drawer; they make him feel shame, not pride.

He left the army because he believed that getting out would solve his problem. But it made everything worse. One of the symptoms of PTSD is hyper-vigilance – a feeling of being on a permanent state of alert. Though that can be useful when you are serving, it's awful when you are trying to be a civilian. Barclay suffers terribly from hyper-vigilance and it causes him to avoid a lot of life. He can't go to the pub because, when he does, he feels he has to keep his back to the wall and an eye on who is coming in. He has to be careful about going to the cinema, as sudden loud noise, especially gunfire, can make him panic; he once fled a Chinese restaurant when other diners starting letting off party poppers. When he moved back in with his parents in Alexandria, he would shout out in his sleep so often that his mother told him it was like a battle going on in the back bedroom.

These experiences are common. There are former soldiers who served in Northern Ireland who, to this day, check beneath their cars for bombs. Often, sufferers of PTSD will turn to alcohol as a means of dampening the hyper-vigilance and nightmares. "When I moved back home, I was heavily self-medicating with alcohol," says Barclay. "I was trying to kill myself with drink."

But booze brings its own problems. "The reason I don't go out now," he says, "is that the last time I did, a year ago, two neds tried to jump me as I was on my way back home. The next thing I remember is a lassie screaming, 'Stop it! You're kicking him in the head!' That scared me because I could have killed the guy. That's my natural instinct – if I feel threatened, I'll explode."

Fiona Manson, a charge nurse at Hollybush House, explains that trauma causes the mind to freeze-frame a certain incident and fail to process it as a normal memory. It's not just visual either. Particular sounds, smells and tastes can all trigger flashbacks; barbecues are a problem for some combat veterans – they're too reminiscent of the smell of burning flesh. From talking to ex-soldiers at Hollybush, it seems to me that it's not just memories that get frozen in time; the men themselves sometimes seem trapped, unable to slough off their military identity and feel at ease in a new peacetime skin.

"When you join the army they break you down and then rebuild you," says Barclay. "They have to do that for you to become able to kill another human being. But when you leave the forces, they don't deprogramme you, so you're still that person. When I came back my family told me that I was completely changed. They say all they want is the old Gavin back."

Barclay pauses to light a cigarette and admire, for the first time in an hour, the incredible view. "I've told them," he says, matter-of-factly, "they'll never see the old Gavin again."

HOLLYBUSH HOUSE is a hulking brute of a mansion, bristling with gables and lums. It's set within 38 acres of grounds, through which burbles the River Doon. The first impression is of peace. The road between Ayr and Dalmellington is back along a tree-lined avenue, and the only sound is the occasional meh and caw of ewes and crows. Hollybush was built as a private home in 1860. Combat Stress took it over in 1986. Apart from a recently built wing, it still feels quite baronial inside. On a noticeboard, a list of DVDs available to watch includes Black Hawk Down, A Bridge Too Far and The Belles of St Trinian's.

Around 1,500 ex-servicemen and women visit here each year, drawn from the army, RAF and navy, staying for a couple of weeks at a time. They come from Scotland, Northern Ireland and the north of England, and range in age from 19 to mid-90s. If you saw them in the communal rooms and corridors, wearing T-shirts, Crocs and shorts, you might think this some sort of holiday camp. It's actually a house full of stories, a remarkable human library, each tale of woe very different yet with a lot of similarities too. Most people who come here have problems with anger, anxiety, hyper-vigilance, hideous dreams, drink, depression and sometimes drugs. They have bad memories so strong that they rear up from the backs of their minds and cast a writhing shadow over weeks and months at a time.

PTSD isn't really something you can cure, especially if you've had it for a while, but you can learn to live with it. While at Hollybush House, the veterans take part in an intensive programme of group and one-to-one therapy sessions on subjects such as anger management and bereavement. Occasionally, therapists and their clients will make trips to the scene of the trauma; Lockerbie, Belfast and the Falklands have all been visited, the intention being to demonstrate that places that were once charnel houses have moved on, and the veteran can too.

One more common treatment is EMDR (eye movement desensitisation and reprocessing), which involves focusing on the memory that is causing distress while moving the eyes quickly from side to side, thus replicating the rapid eye movement that takes place during sleep. It sounds bizarre, but the veterans find this an effective way of making their memories more bearable.

Hollybush also offers treatments of the sort you might expect in a posh spa. "If you asked me when I was in the army what I thought of acupuncture and Indian head massage, I'd have said it's tree-hugging hippie shit," says Garry Walker, a 53-year-old retired major who is in charge of clinical services. "But we've got people 24 hours a day, and to offer them these interventions is very important. A lot of these people will have had no contact with others for months. So no one has touched them in that time. Indian head massage is tactile and breaks down lots of barriers. A lot of guys open up when they get that."

Before arriving at Hollybush at the end of 2003, Walker worked as a psychiatric nurse in the army for more than 20 years. He says that one of the very helpful things about Hollybush is that all the people who come here are from the services. This makes for an environment in which the veterans feel they will be understood – precisely the opposite of how they feel about standard NHS care – but it also creates a certain esprit de corps, even a sense of homecoming.

Among the men, there's a feeling that no one on civvie street can possibly understand what they have seen and felt. So to come to a place where others may also have been in Basra or Belfast is a comfort. It's about no longer feeling isolated. It's about having comrades-in-harm. "If we could take Hollybush home with us, there wouldn't be any PTSD," says 39-year-old Alex Moffat, who served with the Royal Signals in Northern Ireland and the first Gulf War.

It's common for these veterans to be living alone. Their relationships will often have broken up. "I was married," says Donald McLeod, a 48-year-old who served at Tumbledown with the Scots Guards and pulled bodies from the water when Sir Galahad was destroyed, "but when I came home from the Falklands, my wife said I was an animal. She said I went to war young and came back looking old."

The hyper-vigilance seems to spread from combat veterans, in a less intense form, to their families. The wife and children feel wary of the returned man, always aware that the slightest thing could set off his anger. Staff at Combat Stress are aware of this and try to look after the carers too. Stephanie Donnachie, 42, from Kirkmichael, is one of these. She is married to Stevie, also 42, a former Royal Highland Fusilier who was involved in an incident in Northern Ireland in 1983 that was so traumatising – it involved a child – that he has never felt able to tell his wife what happened. Over the course of an hour, Stephanie explains how hard their life has been. They have four daughters, but two have left home, driven out by their father's illness. Stephanie's family no longer speaks to her. Stevie is angry and unreasonable. He shouts and says hurtful things. At one point, he was drinking two bottles of whisky a day.

The breakdown took place about six years ago, before they found Combat Stress and glimpsed a more bearable future. "He locked himself away in his room," Stephanie recalls. "He had been such a capable man and, all of a sudden, I couldn't get him out of his bed, or to take a bath, or to eat. He was just rotting away. And then the final straw was when I caught him with all his tablets. He had taken about half. I took him to hospital and he said, 'Och, it was a mistake. I just took too many.' But it wasn't a mistake. The standing joke in our house is, 'I'm bloody sure you're no' killin' yersel and leavin' me with a' these kids.' You try to make everything light and jokey, but obviously, deep down, it is a worrying thing when you can't trust your husband to be left alone in the house."

Speaking in his office on the top floor of Hollybush House, the wind snuffling at the leaded glass like a curious dog, Walker explains that PTSD often affects people in unexpected ways. The army pushes recruits hard for 22 weeks of basic training, a process intended to filter out people not suited for military life. You are left with a group that is physically and emotionally strong. When things go wrong psychologically, it is often because something happens which they take personally.

Walker tells me about the Graves Registration teams, whose job during the first Gulf War was to recover the dead, sort through their belongings and arrange for them to be transported home. It was distressing work, involving picking up bodies and body parts and putting them in bags, but what traumatised one man was dealing with the men killed in a so-called friendly fire incident. A friend of one of those who died asked whether it would be okay to put a copy of the Rangers News into his pal's pocket as he had been a huge fan of the football team and it was the last meaningful thing he could do for him. This very human request was what broke through the man's psychological defences, wounding him in a way that all the blood and gore had not.

For a number of the men I speak to, it's the same. John Gallagher, 60, fought for the Argyll and Sutherland Highlanders at Aden, but what traumatised him was seeing a man from his regiment die, pinned under an armoured vehicle on the road near Luss.

Alex Moffat shared a barracks room in Northern Ireland with three men, all of whom were killed by the IRA in 1988. What got him, though, was being injected with anthrax vaccine during the first Gulf War. The last thing he remembers was beginning to put his shirt back on when he collapsed with anaphylactic shock. He was rushed to the tented hospital, where he was found to be paralysed. Knowing he would be unable to put on a respirator in the event of a chemical attack, he was put inside a body bag and this was zipped up over his head whenever the warning sounded. Inside the bag, inside his head, he panicked. He lay there looking through the little plastic screen at all the nurses and soldiers running around with guns and gas masks. Sometimes, nurses would sit on either side of him and rub his arms through the bag, trying to comfort him. Though he later recovered from the paralysis, this seems to have gone on for a fortnight, and the cluster of sensations from that time now form the basis for his frequent flashbacks and nightmares.

Moffat, who lives in Larbert, reckons he has had about 150 different employers since leaving the army in 1992. He has trouble concentrating, keeps drifting back to what happened to him, and can't hold a job down. A lot of the men who come to Hollybush are the same. As soldiers, they had a steady income, rank and status. Now, often, they are poor, living in poor quality housing, and held in low esteem by themselves and others.

That's where climbing the Cobbler and other such activities can help; it's a little ego boost and, of course, symbolic – men who have a mountain to climb in their lives literally climbing a mountain. Moffat, who at one time was a world-class skier, climbed it a year ago and considers that a turning point in his life. "At the top, it was as if I'd never had the vaccinations in the first place. I was back where I wanted to be."

He also finds great comfort in Gardening Leave, a charity which works in association with Combat Stress. Anna Baker Cresswell was close friends with a Falklands veteran who committed suicide. Determined to help others who might be going through what he did, she retrained as a horticultural therapist and now invites veterans to help grow flowers, fruit and vegetables in the grounds of the Scottish Agricultural College at Auchincruive. Since the garden is walled and has only one entrance, the paranoid men can relax here. The swooshing River Ayr is also a comfort; therapists at Hollybush teach veterans to deal with intrusive memories by imagining a safe space, and as this often involves the sound of running water, the river's presence reinforces those feelings of safety.

Gardening Leave offers a more subtle therapy too. "People who have PTSD suffer from a sense of foreshortened future," says Cresswell. "As a result of what they have seen and experienced, they can't really look beyond the end of the day. So to engage them in the seasonal cycle of the garden is an almost subliminal way to get them to think beyond today." And, again, there's a symbolism at work. Gardening's themes of death and renewal are not lost on the rootless men who work here. They even grow poppies.

BACK ON THE Cobbler, this time on the way down, I fall in with Robert Thackeray, a 27-year-old from Newcastle who was in the Royal Fusiliers until he was medically discharged as a result of a stress fracture of his right leg and problems with his back. He's walking with the aid of poles.

He seems quite a placid person, gentle even, with pale blue eyes in which lurks unexpected wry amusement. He was part of the build-up of British forces that crossed into Iraq from Kuwait in 2003 as Operation Telic, and he found several things difficult about the six-month tour. His convoy was stuck on a bridge into Basra for eight hours as they came under sustained mortar fire. Once in the city, he was involved in a food-distribution operation, which humanised the enemy and added to his stress levels. Then he was called upon to help dig a mass grave.

PTSD kicked in when he returned to Britain. There was the usual lurid litany of symptoms – alcohol abuse, nightmares, violent behaviour, a suicide attempt. "The thing that kept coming back was a firefight," he says. "Five men in a building were firing out. We dismounted out of a Warrior (armoured vehicle] and returned fire. I saw a bloke coming to the doorway to fire, and I pulled the trigger and watched the three rounds hit his body. I was about 50 yards away, and could see quite clearly through the night-sights. My honest feeling was that I was really scared but I'd rather kill him before he killed me. But later, I saw it in a dream and it kept on coming back. I began to wonder, 'Was I right or wrong to do that?'"

Professor David Alexander, of Robert Gordon University, is Scotland's leading expert on trauma, and a consultant to Combat Stress. He believes that a major psychological pressure on those fighting in Iraq is that the war is not popular back home in the UK, and may not even be regarded as a just cause by the troops themselves. Both Barclay and Thackeray confirm that they didn't believe the British Army should be there, which made fighting much harder.

They also believe that the military ought to be more open about PTSD, perhaps even mentioning it during the recruitment process, and that the government should spend more on treating it; Hollybush House, for instance, gets only 42% of its funding from the Ministry of Defence, and has to raise the rest itself. "Out of the guys who went to the Falklands to fight, more have committed suicide since coming back than actually died out there," says Barclay. "If the military and the government don't do something soon, veterans from Iraq and Afghanistan will go the same way."

Not that the men are bitter about having joined up. In fact, when Thackeray sees coverage of Iraq on the TV news, it makes him want to go back there. As I spoke to veterans for this article, it became clear that they joined because the army offered the possibility of a better life. Gallagher signed up in the late 1960s because he wanted to get away from gang violence in the east end of Glasgow. Thackeray joined in 2000 because he wanted to get away from drug dealers and car thieves in his area of Newcastle. The problem for both men, and for others with PTSD, is that instead of building a future they have ended up imprisoned in the past.

At the foot of the Cobbler, I ask Thackeray what, now, is his greatest dream. "I just want a normal life," he says. "I want to settle down with a family and work nine to five like everybody else." It's a moving answer for being so simple and yet, at this point, so ambitious. One of the goals of the war in Iraq was to win hearts and minds; what a tragedy that an organisation like Hollybush House is needed to help British troops who, in the pursuit of that, have broken and lost their own.

For further information, contact Combat Stress (01292 561300, www.combatstress.org.uk) and Gardening Leave (www.gardeningleave.org)