"I wanted to do something to make my mother proud," Harry, 32, told Tom Bradby of ITV, a U.K.-based television network.
"I never really dealt with what had happened," Harry said, referring to Diana's death in 1997 at age 36. "It was a lot of buried emotion. For a huge part of my life I didn’t really want to think about it."
Sentebale, which translates to "Forget Me Not," was founded in 2006 by Harry and Prince Seeiso of Lesotho, who also lost his mother. The charity helps vulnerable children in Lesotho, Africa, many of whom are struggling with HIV and AIDS.
"There was obviously a connection between us," Harry said in the documentary of himself and Seeiso. "His mother stood for something and my mother stood for something as well."
He added, "To a certain extent there was a lot of unfinished business – work that my mother never completed."
Harry also spoke movingly about how founding Sentebale changed his life and gave him purpose.
"I now view life very differently from what it used to be," he said. "I used to bury my head in the sand, and let everything around you tear you to pieces."
Harry spent a gap year after high school in Lesotho and said he was inspired by what he saw and learned.
"Someone said, 'Right, go to Lesotho,'" Harry recalled. "It’s like where the hell is that?...And now for me, I can see exactly where I want to take it."
Harry described himself today as "fired up" and "energized."
"What started as an idea of me turning round and saying, 'Right, I’ve got a year off. I want to do something really constructive with my life. I want to do something that makes my mother proud. I need to make something of my life,'" he said. "I was fighting the system, going, 'I don’t want to be this person.'"
"My mother died when I was very, very young and I don’t want to be in the position," Harry said. "Now I’m so energized, fired up, to be lucky enough to be in a position to make a difference."
Just last year, Harry expanded the charity's presence in Lesotho by opening the Sentebale Mamohato Children's Center.
Harry named the center’s dining room, "Princess of Wales Hall," in honor of his late mother, who was one of the first pioneers to de-stigmatize the prejudice against those living with HIV and AIDS.
"The fact that I’ve managed to keep Sentebale going...for the last 10, 11 years has been fantastic because now everything else I’m involved with makes sense to me and I’m just getting started," Harry said in the documentary.
Harry also reunites in the documentary with Mutsu Potsane, the little boy who first stole his heart in 2004 when, as an orphan, Mutsu clung to Harry's side.
Harry gave Mutsu a pair of royal blue rain boots at the time, which the toddler cherished and wore to bed. Over the last 12 years, the two have stayed in touch, periodically writing letters, and Harry taking time out to visit him on each trip to Lesotho.
Harry spoke emotionally about how important he and his older brother, Prince William, feel about carrying on their mother's legacy.
"Naturally there’s a crossover of some of the passion between myself and William that the two of us share with the work that our mother started," he said.
"Prince Harry In Africa," courtesy of Big Earth Productions, airs tonight on ITV.