Prince Harry has said it was "only right" to speak about his problems coping with his mother's death because he wanted to encourage others to "smash that stigma" around talking about mental health.
The 32-year-old reflected on his decision to speak openly about the counselling he sought, as he visited the London Marathon Expo to officially open the event and meet charity runners.
The Prince, who was just 12 when his Diana, Princess of Wales, was killed in a car crash in Paris on August 31 1997, said of his mother: "I think from her perspective she would be overwhelmed and hugely encouraged by the fact that the UK - not known for wanting to talk about mental health issues - has suddenly got to this point."
The London Marathon has made the Duke and Duchess of Cambridge and Harry's mental health campaign Heads Together its official charity, and, when he cut the ribbon to launch the Expo - five-year-old Mellisa Howse, whose father Tony Howse is running on Sunday, was on hand to help.
In an interview with the Daily Telegraph, Harry revealed he spent nearly 20 years "not thinking" about his mother's death and eventually got help after two years of "total chaos".
After handing out registration packs to runners which included their official number, timing chip and a Heads Together headband, he said: "I've shared, just as much as everybody else has during this campaign.
"And after how many years of listening to stories from veterans and their families and then specifically in this campaign, William, Catherine and I hearing some of the most heart- wrenching stories based around what people have experienced and then the mental anguish that's happened from then.
"It was only right to share my experiences to hope to encourage others to come forward and smash that stigma, to make it easier for them to talk about their own experiences - so I was just doing my bit.
"When you've heard so many stories from so many other people and if you can relate to that then it's only right that you talk about your own experiences."
William, Kate and Harry's Heads Together initiative has been running for about a year and has been encouraging the public to come forward and speak about their mental health issues or be a sympathetic ear for someone in need.
They have been intensifying their campaign in the run up to the London Marathon, and hoping the thousands of runners who will be pounding the streets of London on Sunday will be wearing their blue Heads Together headbands.
Harry was asked whether he was in a good place as a result of dating US actress Meghan Markle but he replied: "The point that we've learnt over this campaign is that if you're able and comfortable enough to be able to talk about certain issues, certain experiences, then you come out of it a far better person."
But he later conceded he was in a good place.
When questioned about his interview with the national newspaper it was suggested the Queen, his grandmother, would not have opened up in a similar way.
Harry replied: "It was the right time to have that conversation and the right way to have it."
He went on to say: "Every single person that we've met has come to us and shared - we've been asking everybody else to share.
"Now if there's any way that our experiences in the past can help with that and help other people come forward, and if our experiences and sharing those experiences can help reduce the stigma for the rest of the UK - then that's where the duty and service bit comes in."
William, Kate and Harry will be at the London Marathon on Sunday but will not be running, something they thought about.
Harry said: "Yes all three of us were tempted but it was probably safer and easier for us not to, and to try and do our best to lead the campaign from the side and let the focus be on the Heads Together runners, which was 200, but is now 700."
Asked what could be done about the "politically sensitive" subject of a lack of mental health services, Harry replied: "Of course it's politically sensitive."
He added: "It's not our job to make those changes, our job specifically for this campaign is to remove the stigma, to allow the conversation to happen."