Prince Harry turned to drugs and alcohol to "mask" the pain he felt in his late 20s.
The 36-year-old royal - who has two-year-old son Archie with pregnant wife Meghan, Duchess of Sussex - admitted he used partying as a way to escape his struggles, and carried on even when he wasn't enjoying himself anymore.
Speaking on his new TV show 'The Me You Can't See', he said: "I was willing to drink, I was willing to take drugs, I was willing to try and do the things that made me feel less like I was feeling.
"But I slowly became aware that, okay, I wasn't drinking Monday to Friday, but I would probably drink a week's worth in one day on a Friday or a Saturday night. And I would find myself drinking, not because I was enjoying it but because I was trying to mask something."
Harry - who admitted the "happiest time" in his life was the decade he spent in the Army - also told how he suffered from "panic attacks and severe anxiety" on royal duties during those years, which he admitted were a "nightmare time" for him as he was always in "fight or flight mode" and "freaking out".
He said: "Every single time I jump in the car and every single time I see a camera. I would just start sweating. I would feel as though my body temperature was two or three degrees warmer than everybody else in the room. I would convince myself that my face was bright red and that everybody could see how I was feeling, but no one would know why. So it was embarrassing. You get in your head about it...
“Everywhere I go every single time I meet someone it’s almost like I’m being drained of this energy picking up on other people’s emotions, finally I would bump into somebody who was sweating more than me and I would stop be able to speak to them and everything would calm down and then I could move on again."
After he met the former 'Suits' actress, Harry knew he needed to have therapy to "fix" himself or he would "lose this woman who I could see spending the rest of my life with."
And it was Meghan who urged him to get help, which led to him confronting the emotions he'd kept buried since his mother, Princess Diana, died when he was just 12 years old.
He said: "There was a lot of learning right at the beginning of our relationship. She was shocked to be coming backstage of the institution of the British royal family.
"When she said, 'I think you need to see someone,' that was in reaction to an argument we had. And in that argument, not knowing about it, I reverted back to 12-year-old Harry.
"The moment I started therapy and probably within my second session, my therapist turned around to me, and said, 'That sounds like you are reverting to 12-year-old Harry.' I felt somewhat ashamed and defensive.
"She said, 'I'm not calling you a child. I'm expressing sympathy and empathy for you for what happened to you when you were a child. You never processed it. You were never allowed to talk about it. And all of a sudden now, it's coming up in different ways as projection.'
"That was the start of a learning journey for me. I became aware that I'd been living in a bubble, within this family, within this institution, I was sort of almost trapped in a thought process or a mindset."
And through therapy, the prince - who stepped back from royal life last year - learned one of the "biggest lessons" he's ever had.
He said: "You've sometimes got to go back and to deal with really uncomfortable situations and to be able to process it in order to be able to heal. For me, therapy has equipped me to be able to take on anything. That's why I'm here now. That's why my wife is here now."