Princess Diana was reportedly at her wit’s end when she decided to put her trust in a British journalist to tell her heart-wrenching story that involved a reported secret battle with bulimia, suicide attempts and a husband who couldn’t give up his mistress.

Andrew Morton penned the 1992 biography “Diana: Her Story,” which was based on the secretly recorded conversations between the princess and her friend, James Colthurst, before her divorce from Prince Charles in 1996. And even 20 years later, after her death at 36 in a 1997 car crash in Paris, Morton still wonders why she chose him.

“It’s a question that's always perplexed me,” the 64-year-old told Fox News. “[But] she knew I was sympathetic to her. She knew I was writing a biography on her. She knew I knew some of her friends… She tested me out with a couple of stories to see how I’d do… And the fact that I was independent of newspapers, of television meant that I could do her bidding… And I think all of those factors came together, with a deep desire to tell her story.”

Morton recalled his experience in TLC’s upcoming documentary “Princess Diana: Tragedy or Treason?” It is one of several specials airing to coincide with August 31, the date of Diana’s passing. He explained questions for the royal were written out and then given to Colthurst, who reportedly passed them on to Diana. She would then record her responses and the tapes were used to help Morton write his book.

“It was a sign at that time [of] this desperation to get the story out,” explained Morton. “I asked her [why] and she just felt the public didn’t really know who she was. They were responding to a two-dimensional image. This kind of media cut-out… she felt like she was enduring a lonely miserable life inside the palace and outside, she was adored… It was incredibly frustrating as far as she was concerned because everyone still believed in the fairy tale. And she knew it was a nightmare.”

Even before Diana married Prince Charles in a televised wedding that attracted about 750 million viewers worldwide in 1981, she long suspected he was having an ongoing affair with his ex-girlfriend Camilla Parker Bowles, Morton said.

“Here is Prince Charles telling Diana that he’s going to give a bracelet to his old lover just before the wedding,” said Morton. “Just a few days before the wedding. And then keeps pictures of Camilla in his diary on their honeymoon. What woman is not going to be incensed by this crass and insensitive behavior? Diana was very suspicious of Charles’ relationship with Camilla right from the get-go. And who could blame her? He left her behind, but the fact that he’s been [currently] married for 12 years to Camilla shows that he still had plenty of love in his heart for his former mistress.”

When it came to Charles loving Diana, Morton seemed unsure.

“Diana said to me that when he asked her to marry him… she said, ‘Oh yes, yes I will. I love you so much.’ And Prince Charles, even in the privacy of that moment, said, ‘Whatever love means.’ And he gave that famous television interview, ‘whatever love means.’ So you have to ask yourself, did he really have any kind of genuine feeling for Diana or was she, as she felt herself, a sacrificial lamb…producing an heir and a spare and then being discarded?”

Morton added Diana quietly endured the collapsing marriage and constant attempts to fit in with the royal family because she was struggling with an eating disorder, as well as the pressures that came with a high-profile role, where every public engagement was scrutinized by the press. Somehow, he claimed, she continued to hope things would get better with time.

“[Diana] was hoping she would come to terms with this new life and move on,” he said. “But she realized she was living this lie… seemingly the wife of Prince Charles. And yet you got Prince Charles, who was effectively with another man’s wife at Highgrove, their country estate, while she languished alone at Kensington Palace.”

There was also one memory in Morton’s book that he insisted left her in tears, which hinted she was also suffering from depression.

“It was a chapter where she was talking about what she called the ‘dark ages’ and how worthless she felt,” he explained.

However, Diana found a sense of happiness in her marriage with the birth of her two sons. The former couple welcomed Prince William in 1982, followed by Prince Harry in 1984. Morton claimed that while Diana was pregnant with Harry, she and Charles experienced an intimate connection, one that involved him reportedly writing love notes to his wife. But after Harry was born, the relationship soured until they finally divorced.

But even while Diana coped with the end of her marriage, she relied on her sons for support.

“As the boys got older and became like her counselors and friends as sons, she began to enjoy life a lot more,” said Morton. “Diana was very protective of William and Harry. [If] you ever criticized the boys... she would be on you like a tigress. She was the only one who could criticize those two. Of course, she indulged them… She wanted to be a full hands-on parent herself. And interestingly, Prince William recently said the same about his own children, George and Charlotte. He wants them to enjoy a relatively normal upbringing.”

The sons would tragically lose their mother when William was 15 and Harry 12. TLC’s documentary explores the many conspiracy theories of what really caused Diana’s sudden death, but Morton said the explanation is a simple one.

“Well, I think the paparazzi followed her all of her adult life, so they contributed to her death,” he said. “[But] it wasn’t their fault. It was the fact that she was being driven too fast… by a man who was found with drugs and drink. It’s the banality of her death which I think most people can’t come to terms with.”

But two decades later, Morton said there’s still plenty to discover and appreciate about “the people’s princess.”

“I think what I did realize was that she left an awful lot of her life in compartments,” he explained. “And you felt like you knew her, but you didn’t. You knew a bit of her… She was a mysterious woman as well as being a compassionate woman.”

"Princess Diana: Tragedy or Treason?" premieres July 31 at 8 p.m.