A tall slim figure clad in a stylishly pretty blouse and skirt, with carefully styled hair and expertly done makeup, Delia Johnstone attracts more than a passing glance. But a look of puzzlement swiftly crosses the faces of the interested, as they realise that this willowy, elegant woman is not quite what she seems.

"It's understandable," says 55-year-old Delia. "I'd be the first to admit that I'm what you might call a work in progress."

Only the 6,000 or so transgender people in the UK can imagine what it means to wake up every single day utterly and absolutely convinced that you are in the wrong body. To feel that every characteristic of your gender is alien – and that in your case, Mother Nature got it terribly wrong. This feeling of not belonging is something Delia lived with for most of her life.

The case of Lea T, the young transsexual model starring in the new Givenchy adverts, perhaps shows that it is getting easier for transsexuals to be out and proud. Gender dysphoria is a recognised diagnosis and trans people can expect sympathy and help from their doctors. But still, living with the sense that you belong to the other gender can be intolerable, and the suicide rate for those with gender dysphoria is shockingly high – about 40 per cent.

It is because of the lack of understanding and support that Delia, who until relatively recently was David, has decided to go public about the change she is going through.

"From a very young age – probably eight or so, I knew that I was very different from other boys. Whenever I looked in the mirror, I hated what I saw, but I had no idea why I felt so bad about myself," she says. "My gran was the only person who seemed to realise something was wrong. I used to go and see her for tea every Wednesday, and every now and then she'd look at me and say, 'You're not who you are'. I knew she was right, but I didn't know why. I also didn't understand my fascination for my sister's clothes and why, a couple of times, I felt compelled to sneak into her wardrobe and try things on."

David was also struggling with undiagnosed dyslexia, and the frustration of being labelled as stupid by teachers who'd never heard of the condition, combined with feeling "all wrong" made him rebellious and badly behaved. He was expelled from school and sent to a council-run boarding school for difficult boys – which Delia describes as one step away from a borstal. Within an hour of David's parents leaving him there, he had been attacked with a pen knife (he still bears the scar), and the careless violence that typified daily life at the school taught him to build a protective shell around himself which served as a defence mechanism for years to come.

Although he hoped desperately that one day he would wake up and feel "normal", the sense of wrongness persisted. Delia remembers a week when David was at home alone – his parents and brother were away – during which he spent the entire time dressed as a woman. "For the first time in my life I relaxed and just felt like me," she says. "And that's when I realised there was more to all this."

Fast cars and motorbikes, and a successful career in digital communications went some way towards providing an outlet for David's pent-up aggression and frustration. And by the time he was in his twenties, David had discovered he wasn't, as he'd assumed, the only person ever to feel like this. But he had also found, from reading tabloid newspapers, that people like him were labelled as freaks and aberrations of nature, and his feelings of shame were almost overwhelming. He decided he must accept the awareness of being in the wrong body and became grimly resigned to the fact that eventually life would become unbearable, and the only way out would be to kill himself.

People might wonder why, convinced as he was of his gender mis-assignment, David got married and had two children. Delia's answer is straightforward, and loaded with the tragedy of a life largely wasted in pretence. "I thought I was in love – I wanted to be in love, but I now realise I just wanted a best friend," she says. "We got married in St Lucia, on the beach. It was beautiful and very romantic, and as I looked into my future wife's eyes and said my vows, I knew I was lying to her. But I felt I had no choice. And I knew that if I tried to tell her how I felt, she would be disgusted, and leave me." The couple struggled with their sex life – Delia remembers wryly that "often I couldn't even raise a smile" and that David would do anything to avoid making love – but they still had two children, a son and a daughter.

Fortunately, the torment within did not interfere with David's feelings of love and commitment for the children. "I was a good father," Delia says firmly. "I adored my children, and we had, and still do have, a close relationship." This is borne out by the fact that when the marriage, which had been rocky for a long time, broke down two years ago, the children chose to live with their father. And it was his devotion for them which, in effect, saved her life.

In September last year, just after the children went off to university, "things came to a head for me," she says. "I'd got to the point where I could no longer live as I was, but research shows that most people who choose to pursue a gender change lose everything. Jobs, families – entire lives. Accordingly, my options seemed limited. I knew that taking my own life would obviously be dreadful for the children. But, it seemed, not as dreadful as discovering the truth about me."

David planned his suicide with meticulous precision, even allowing for a change of mind, which meant that thankfully, at the last minute, he was able to abort the act. "Even as the cord was tightening around my neck and I was blacking out, I was becoming aware of the realisation that perhaps the other way could work. And maybe the kids could accept me after all."

Unsurprisingly, this epiphany triggered the action of purchasing female hormone drugs on the internet. And three months later, in December last year, David finally summoned the courage to visit his GP, who was sympathetic, and immediately organised a supply on the NHS. Delia' s newly soft and unlined skin, a by-product of the female hormones she takes, reduces her age appearance by at least 10 years. Making it easy to forget that when David was growing up, in the Fifties and Sixties, the idea of changing gender would have been regarded as an aberration. Even now, she remembers the terror of being thought of as a freak. And never more when, beset with nerves, David realised that it was time to break the news of his changing status to the children. "The hormones I'd been taking were acting much faster than I'd thought they would. In the three months that the kids had been away at university, I'd grown small but noticeable breasts – and I knew the time had come to explain everything," says Delia. "But I was scared. Who wouldn't have been? I was about to tell my children that the dad they'd loved and looked up to all their lives was becoming a woman."

She now knows that when David asked his children to meet him about something important, they thought he was going to tell them he was gay. His daughter refused to meet him, but his son accepted the news with reasonable stoicism, and left to tell his sister. Delia describes their attitude now as "still struggling" but says both children remain supportive and loving. "They were proud of me once, and I think they'll be proud of me again," she says.

Current NHS guidelines are that people who have been diagnosed with what's called gender dysphoria, should be referred to a specialist gender identity clinic, where support and advice are on offer, as well as hormone treatments and therapies including hair removal and speech therapy. The next step is real life experience (RLE) which means living as your preferred gender for at least a year. Only then are you considered for surgery to finalise the gender transition. A review of studies done in the last 20 years found that 96 per cent of people who'd had gender reassignment surgery were happy. And it seems that Delia is well on the way to joining their ranks. After telling the children, just before last Christmas, extended family, friends and colleagues were also told. Reactions have included astonishment and disbelief, but all, ultimately, have been supportive, and comments such as "we admire your courage", have not been uncommon.

At 12.10am on 1 January this year, Delia hugged some friends at a New Year's Eve party, and walked away to begin her new life. Since then, she has changed her name by deed poll from David Francis Johnstone to Delia Frances Johnstone, and has dressed and lived as a woman. She will continue to take female hormones, and has a painful three-monthly injection of male-suppressing hormones. Eventually she hopes to have the surgery which will finalise her transition.

Delia is very clear that her main motive for speaking out publicly is to let others in her situation know that they are not alone, thereby avoiding some of the agony that David went through in the early days of self discovery. She has recently launched a website that will provide every possible kind of emotional and practical support for transgender people.

"Changing sex isn't just a matter of taking hormones and having an operation," she says. "Women and men talk and move differently, they have different mannerisms and different facial expressions. I can refer people to voice coaches, deportment trainers, and experts on dress, hair and makeup, all of whom who have helped me immensely, and continue to do so. It's a long and difficult journey and there's a lot to learn along the way. I've covered a great deal of ground, but I've still got a way to go. As I said, I'm a work in progress, but I'm happier now than I've ever been."

The changing face of sex reassignment

- Public intolerance towards transsexualism continues to exist widely, but when a beautiful young transsexual such as Lea T – the world's first transsexual supermodel – poses in Vanity Fair and French Vogue, it perhaps shows that progress is being made. Givenchy's autumn/winter collection will feature Lea, once a personal assistant for the fashion brand.

- Lea, formerly Leandro, is the daughter of the former Brazilian footballer Toninho Cerezo. She has had a difficult journey from a Catholic childhood to going public and acknowledges that her problems are far from over, especially given intense media interest in her. She is currently undergoing hormone treatment to give her the body of a woman.

- The world's first male-to-female sex operation was performed in Dr. Magnus Hirschfeld's Institute for Sexual Research in Berlin in 1930, before the Nazis closed down the centre in 1933.

- In 1945, the London-based surgeon Sir Harold Gillies carried out the world's first female-to-male sex change on Michael Dillon, formerly Laura, after the trial usage of testosterone gave Michael the appearance of a man.

- Gillies, known as "the father of plastic surgery", monitored the effects of the hormone drug before opting to operate – an approach that is used today in the NHS Real Life Test. Transsexuals are expected to live as their preferred gender for at least a year and, while some cosmetic surgery is permitted, gender confirmation surgery is delayed during this period.

- A landmark 1999 Court of Appeal decision to uphold a High Court ruling that recognised Gender Dysphoria as a legitimate illness granted the right to sex change operations on the NHS. The Gender Recognition Act of 2004 then allowed transsexual people to obtain legal documentation stating their preferred gender, including a new birth certificate and passport, as well as the right to marry in their preferred sex.