Prince Harry said he only started to process the trauma of losing his mother after meeting wife Meghan.
The duke, 36, said he feared his relationship wouldn’t last unless he started to deal with the past.
Harry was just 12 when Diana died in a crash as she fled paparazzi in Paris in 1997.
But he only started therapy for his grief and anxiety four years ago.
Harry said: ‘I quickly established that if this relationship was going to work that I was going to have to deal with my past.
‘If we hold on to grief it manifests itself and appears later in life. That is what I’ve learned from this process.’
The duke added that starting the Invictus Games for injured veterans in 2014 was another important part of his healing process.
‘I think that compassionate element, putting ourselves in someone else’s shoes and help in that healing process is absolutely critical,’ he said.
Harry made the comments during a surprise appearance on Good Morning America to promote his new documentary, The Me You Can’t See.
The programme, about mental health, launched on Friday morning and saw Harry make more bombshell claims about the Royal Family.
In it, Harry accused the firm of ‘total neglect’ and said at times ‘life was a nightmare’.
He claimed that his family did not speak about the tragic loss of Diana and just expected him to deal with the resulting press attention and mental distress.
He said he initially coped by not thinking or talking about Diana but later was ‘just all over the place mentally’ and turned to drink and drugs.
Things turned a corner when he met former actress Meghan, who encouraged him to go to therapy after an argument.
‘I knew if I didn’t do the therapy and fix myself that I was going to lose this woman who I could see spending the rest of my life with,’ he said in the documentary.
‘When she said, “I think you need to see someone,” it was in reaction to an argument we had. In that argument, not knowing about it, I reverted back to 12-year-old Harry.’
Harry has been using eye movement desensitization and reprocessing therapy (EMDR) to work through the death of his mum, his feelings about the media and the online harassment and abuse him and his family have received.
The technique is usually used to help someone deal with a traumatic event by focusing on it.
Harry said he still struggles with ‘the clicking and the flash of cameras’, which ‘takes me back to what happened to my mum, what I experienced as a kid’.
However, he said therapy has equipped him ‘with the ability to take on anything’.
The Me You Can’t See is a five part series he co-produced with friend and American TV host Oprah Winfrey.
The documentary also hears from Zak Williams, son of late comic Robin Williams, who took his life in 2014.
During his Good Morning America appearance, Harry drew parallels with Zak’s experience of grief.
He said: ‘Zak’s story is a classic example, sort of the parallel thread between what happened with him at his age and what happened to me at my age. It’s remarkably similar.
‘There was one thing he said in a follow-up conversation which is his service to others has helped heal him and I think that was a really key moment for Oprah, myself and Zak when we were on a separate call to go, “wow, it’s true”.
‘Oprah, in your career, it’s been true for me and starting the Invictus Games to be able to create something to watch other people heal is part of our own healing process.’
Harry said he hoped speaking about his experience could help others, adding: ‘[There are] so many people of all ages that need to heal and that also are for one reason or another unable to heal or maybe unaware that they need to heal.’
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