The 32-year-old told the Daily Telegraph: "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'.
"So I was 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 shut down his emotions after her death which had "a quite serious effect on not only my personal life but my work as well".
The prince sought help 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."
Last year The Huffington Post UK collaborated with Prince William and Kate on the 'Young Minds Matter' project which sought solutions for the stigma surrounding the mental health crisis amongst children in the UK.
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."
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."
He added that he could "safely say" his concerns were not related to his service as a soldier in Afghanistan.
Prince Harry also spoke of how boxing helped him. He 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."
His frank interview has been widely lauded and it is hoped will remove some of the stigma still present around mental health.
More people are having "life-saving" conversations about their mental health - but men and older people are far less likely to do so, a landmark survey found last month.
Nearly half of us have discussed mental health in the last three months - which was described as a "tipping point" for the issue - and 82% say discussing their own issues was helpful, according to YouGov's poll, the most comprehensive survey of its kind for 20 years.
The gulf between the sexes is large, with only 37% of men saying they discussed mental health compared with 54% of women. A total of 57% of those aged 18 to 24 had, compared with just 32% of those over 65.
The films feature people talking about their mental health, often with the person they first opened up to.