It was the image that all-but broke the hearts of the British nation: the small bewildered boy, just 12, walking behind his mother's coffin as the world watched on.

With clenched fists and head bowed, the image of a stoic Prince Harry became the enduring memory from the funeral of Diana, Princess of Wales, moving onlookers to tears.

Only now, 20 years on, will those same onlookers come close to understanding the long-term effect the cataclysmic news of his mother's death had on the boy prince, due to his own candid disclosures.

The courage he showed on the cusp of his teenage years is mirrored today as he allows strangers a glimpse into his difficult past in an honest and wholehearted effort to help others suffering mental health issues.

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His late mother once pledged to come running to anyone who found themselves in distress, calling it her destiny to help others less fortunate than herself.

Her younger son, now 32, could not be following more closely in her footsteps.

He said: "What my mother believed in is if you are in a position of privilege or a position of responsibility and if you can put your name to something that you genuinely believe in... then you can smash any stigma you want."

The stigma in question is mental health: a cause that Harry, his brother and sister-in-law, the Duke and Duchess of Cambridge, have made their flagship issue of the past year.

Having spoken passionately about the importance of conversation, the Prince has many times implored others to speak about their worries, lest they fester and grow.

But even as he campaigned, his relative (and understandable) silence on his own past has served only to leave a vacuum for speculation about what specifically had inspired him.

Had he suffered trauma from active service in Afghanistan, some wondered, or was he struggling to define his role in his privileged but deeply unusual life?

In fact, as we now know, it was his mother's death which left Prince Harry in a state of emotional crisis waiting to happen, swallowing his feelings and determined not to confront the trauma that had riven his young life.

Born in 1984, the younger son of Diana and the Prince of Wales quickly found a special place in the nation's hearts.

Many recall with fondness photographs of him pressing his tongue against a car window, giggling and soaked while riding a log flume, or learning to ski.

Then, on August 31, 1997, Prince Harry and his brother William, 15, woke at Balmoral at 7.15am to learn that their mother had died.

At the funeral, the princes walked behind their mother's coffin with the support of their father, grandfather and uncle, Earl Spencer.

The princes then attempted to return to normal life: William at Eton and Harry to Ludgrove.

Harry's biographer, Penny Junor, later noted: "Whether by nature or nurture, both boys had even then that Windsor ability to keep their emotions in check".

Says the Prince now: "I have spent most of my life saying 'I'm fine'."

Princess Diana.
Princess Diana.

In his twenties, he says, he made a valiant stab at pretending all was well, convincing himself "life is great" as he embarked on being an adult royal. As he describes it, there followed two years of "total chaos".

Any transgressions of his youth - an ill-judged fancy dress costume and intimate photographs taken at a private Las Vegas party - have largely faded from the public consciousness.

In hindsight, and after years of public service, they have become part of his man-of-the-people persona: the high jinks of a well-meaning but careless twentysomething better suited to military banter than royal protocol.

But, with today's interview, a more complicated picture emerges.

"I, through a lot of my twenties, was a problem and I didn't know how to deal with it," he says. "I was a typical 20, 25, 28-year-old running around going 'life is great', or 'life is fine'... and then started to have a few conversations and actually all of a sudden all of this grief that I have never processed started to come to the forefront and I was like, there is actually a lot of stuff here that I need to deal with."

Despite those difficulties, a stellar military career saw him find his feet at Sandhurst as Officer Cadet Wales, joining the Household Cavalry (Blues and Royals) as a second lieutenant and deployed secretly to Afghanistan for 10 weeks in Helmand Province.

In doing so, he became the first member of the Royal family to fight in a war since his uncle, the Duke of York, in the Falklands.

At the time, Prince Harry said: "It's very nice to be sort of a normal person for once, I think it's about as normal as I'm going to get."

His active service and hands-on approach endeared him to colleagues, superiors and the public.

In 2012, after qualifying to fly an Apache helicopter, he returned to Afghanistan for a 20-week deployment, then came back to London to throw himself into the Invictus Games in 2014.

A year later, after a decade with the Armed Forces, it was announced he would be leaving operational service with the rank of captain to concentrate on his charity work and duties for the Queen.

His time with the Army has left its mark, not least in the way he deals with adversity. He is a "massive believer" in humour, he said, adding: "If you sit down and talk to most of these guys about the issues that they have had, it's all dark humour.

"I can safely say in Headley Court, when these guys go through their rehab, the idea of someone being stuck in bed and being late for breakfast because someone's nicked both of their prosthetic legs is part of how you get through it for them.

"I'm not suggesting it's how the rest of the general public do that, because it probably won't work and I will get crucified for it, but in that specific experience that I've had, it works for those guys."

Two years with the personnel recovery unit followed, where Prince Harry worked to improve the welfare of wounded, injured and sick servicemen and women, including those with serious mental health issues.

The job, he says, required him to park his own issues to listen to others. In just one day, he worked with a girl who had tried to commit suicide, a man suffering from such severe PTSD he could not speak, another who had to sleep with a rain sound effect to mask tinnitus caused by a grenade, and a group of terminally ill children.

"All you want to do is help and listen but then you walk away going 'hang on a second, how the hell am I supposed to process this?"' the Prince said.

As he took on more public royal engagements, he admits suffering a "flight or fight" reaction, explaining: "Being in situations when you're at an engagement and not being able to do the flight bit, your body ends up kicking into the fight."

Prince Harry with Prince William, visiting the Canadian National Vimy Memorial in Vimy, near Arras, northern France. Photo / AP
Prince Harry with Prince William, visiting the Canadian National Vimy Memorial in Vimy, near Arras, northern France. Photo / AP

IN HIS OWN WORDS

'Losing my mum at the age of 12 and therefore shutting down all of my emotions for the last 20 years has had a quite serious effect on ot only my personal life but my work as well

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What we are trying to do is normalise the conversation to the point that anyone can sit down and have a coffee and just go, you know what, I've had a really s*** day, can I just tell you about it? because then you walk away and its done. Rather than a week later or 20 years later, what could have been something small could grow into this beast of a snowball which you can't dislodge.

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The best or the easiest people to speak to is a shrink or whoever... I've done that a couple of times, more than a couple of times.

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I can count myself very lucky. It was 20 years of not thinking about it and two years of total chaos.

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My way of dealing with it was sticking my head in the sand refusing to ever think about my mum, because why would that help? It's only going to make you sad, it's not going to bring her back. So from an emotional side I was like, right don't ever let your emotions be part of anything.

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I have probably been very close to a complete breakdown on numerous occasions when all sorts of grief and sort of lies and misconceptions are coming to you from every angle. But you know, it comes with the job, it comes with the role, and one of the hardest things, I suppose, is not being able to have that voice or being able to stand up for yourself.

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My brother... was a huge support to me. Kept saying "this is not right, this is not normal, you need to talk to about stuff, its OK."

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Once I offload my stuff to somebody else I feel so much better. I know there is huge merit in talking about your issues and the only thing about keeping it quiet is, it's only ever going to make it worse.

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Prince William, left, Kate Duchess of Cambridge and Prince Harry wearing charity headbands. Photo / AP
Prince William, left, Kate Duchess of Cambridge and Prince Harry wearing charity headbands. Photo / AP

In charity work, he has always seemed to find his calling. He became a high-profile champion for veterans, taking part in a Walking With the Wounded charity trek to the South Pole and helping to make Help for Heroes a global success.

Sentebale, founded with Prince Seeiso of Lesotho in memory of their mothers, has gone from strength to strength raising money for orphans and vulnerable children. His Invictus Games has become a byword for achievement in adversity, giving new hope to those who have sacrificed their health for their country and enlisting the Obamas and the Queen in its promotion.

"You know, for me, Invictus would have never got off the ground if I hadn't dealt with all that stuff beforehand," he concedes now.

Notably, he also embraced charities close to Diana's own heart, picking up her mantle with campaigns against landmines and for HIV awareness as well as his own love of conservation and sport.

For years he has been praised for his natural way with children, employing his own juvenile side to win their confidence in all sorts of circumstances.

In Harry's relationships, too, things seem to have settled. Past girlfriends had endured huge intrusion into their private lives after being photographed on the arm of the Prince.

Those watching from afar empathised with a man who wore his heart on his sleeve, but could not replicate the success of his brother's match with Catherine Middleton.

But the arrival of Meghan Markle, a 35-year-old American actress, has fired hopes for his personal happiness. A charity campaigner, yoga lover and - until recently - lifestyle blogger, Markle is already well used to the spotlight and is committed to just the philanthropic causes Prince Harry admires.

Anyone who doubted the strength of feeling in the fledgling transatlantic relationship would have been reassured by a ferociously protective letter authorised by the Prince, railing against a "storm" of abuse and harassment.

"Of course, I would love to have kids," he says.

He is, he says, in a "good place". As he puts it: "I am a prince, I have a house over my head and the security that I need. I have a car, I have a job that I absolutely love.

"Previous to that I had a second job that I absolutely loved as well. I now, because of the process I have been through over the past two and a half years, I've now been able to take my work seriously, been able to take my private life seriously as well and be able to put blood, sweat and tears into the things that really make a difference."