The 32-year-old Royal said he nearly had a total mental breakdown several times, but his brother the Duke of Cambridge stepped in when he needed him most.
He said he was not ready to seek help for himself and, without his brother, Prince Harry doesn’t know how far he could have sunk.
Prince Harry, who was William’s best man when he married the Duchess of Cambridge in 2011, said: “It’s all about timing and for me personally, my brother, you know, bless him, he was a huge support to me.
“He kept saying this is not right, this is not normal, you need to talk to someone about stuff, it’s OK.
“The timing wasn’t right.
“You need to feel it in yourself, you need to find the right person to talk to as well.”
Prince Harry reveals his brother, William, forced him to get mental health help Royal siblings: Prince William and Harry Thu, September 15, 2016
A round-up of the brother-royals, Prince Harry and Prince William.
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Royal siblings, Prince William and Prince Harry in pictures
Prince Harry said he spiralled into two years of “total chaos” in his twenties and never emotionally dealt with the death of his mother, Princess Diana, who was killed in a car crash in Paris.
The Ludgrove-educated Royal would not allow himself to think about his mother, because “why would that help?” – but now realised shutting it out was not the answer.
He added: “I can safely say that after losing my mum at the age of 12, and therefore shutting down all of my emotions for the last 20 years, has had quite a serious effect on not only my personal life but my work as well.”
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Prince Harry's turmoil began when his mother did in a car crash in Paris
But the time is right for Prince Harry to work with Prince William and the Duchess of Cambridge because “as long as we’re this age and we’re still interesting then we want to make as much of a difference as we can for the better”.
Prince Harry said he wants to provide a platform for people to open up about their mental health issues so they do not bottle it all up inside and implode like he did.
The brothers have always had a close relationship and Prince Harry is grateful to WIlliam
He said: “What we are trying to do is normalise the conversation to the point where anyone can sit down and have a coffee and just go ‘you know what, I’ve had a really s*** day, can I tell you about it?
“Because then you walk away and it’s gone.”