Prince Harry has revealed he sought counselling after spending nearly 20 years "not thinking" about the death of his mother, Diana, Princess of Wales.
In an interview with the Daily Telegraph, he said it was not until his late 20s that he processed the grief - after two years of "total chaos" and coming close to a "complete breakdown".
But he was in a "good place" because of the "process I have been through".
Harry, 32, also said boxing had "saved" him by helping him let out aggression.
The Telegraph says Prince Harry had decided to talk about his past in the hope it would encourage people to break the stigma surrounding mental health issues.
Along with his brother and sister-in-law, the Duke and Duchess of Cambridge, he is promoting the Heads Together mental health campaign, the London Marathon's charity of the year.
Speaking to the paper's Bryony Gordon, Prince Harry said: "I can safely say that 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 not only my personal life but my work as well."
He added: "I have probably been very close to a complete breakdown on numerous occasions when all sorts of grief and all sorts of lies and misconceptions and everything are coming to you from every angle."
By Peter Hunt, BBC royal correspondent
This is a senior royal as we rarely see or hear them.
Prince Harry is part of the future of an institution that once embodied the British stiff-upper-lip approach to life.
Such an approach was dealt a blow by Diana, Princess of Wales.
In her 1995 BBC Panorama interview, she told Martin Bashir about her post-natal depression, her self-harming and her bulimia.
Now, Prince Harry is following his mother's example.
In the podcast, which is a compelling listen, Harry admits to feeling nervous as he speaks openly about suffering from anxiety, coming close to a breakdown and being a "problem" for much of his 20s.
Prince Harry is this country's most high profile person yet to talk about his personal mental anguish.
His privileged life in a palace hasn't protected him from ill-health.
Diana's son hopes that his up-front display of honesty will help to break the taboo that still surrounds mental health.
The Princess of Wales died in a car crash in Paris in August 1997.
Prince Harry said: "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?"
"(I thought) 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.'"
He described himself as a "typical 20, 25, 28-year-old running around going 'life is great', or 'life is fine' and that was exactly it.
"And then [I] 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."
He said he decided to act after his brother - the Duke of Cambridge - told him: "Look, you really need to deal with this. It is not normal to think that nothing has affected you."
Prince Harry opened up to the Telegraph's Bryony Gordon over a cup of tea at Kensington Palace during a half-hour interview recorded for a podcast on mental health issues.
Gordon, who has previously spoken of her own struggles with bulimia, alcoholism and an obsessive compulsive disorder, said the pair had been alone in the room.
She said it had been unusual to hear a royal talk for half an hour on any subject and described Prince Harry as "really articulate and a sound guy".
Prince Harry said: "Some of the best people or easiest people to speak to is a shrink or whoever - the Americans call them shrinks - someone you have never met before.
"You sit down on the sofa and say 'listen, I don't actually need your advice. Can you just listen'. And you just let it all rip."
'Let out aggression'
Asked whether he had counselling, he said: "I've done that a couple of times, more than a couple of times, but it's great."
But he said he could "safely say" his concerns were not related to his service as a soldier in Afghanistan.
On taking up boxing, Prince Harry told the paper: "Everyone was saying boxing is good for you and it's a really good way of letting out aggression.
"And that really saved me because I was on the verge of punching someone."
Prince Harry said: "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 been able to put blood, sweat and tears into the things that really make a difference and things that I think will make a difference to everybody else."
A two-part series, Mind Over Marathon, starts on BBC One at 2100 BST on Thursday 20 April, as part of a Minds Matter series of programming about mental health issues.