"When I was 24, 25, I fell deeply in love with somebody who worked in this environment... I was quite happy to give all this up... just to go off and live with him," she said.
"Can you believe it? And he kept saying he thought it was a good idea too."
NBC said the object of her affections was Barry Mannakee, a married father of two who died in a motorcycle accident in 1987.
"And he was the greatest fellow I have ever had. I was like a little girl in front of him the whole time. Desperate for praise... I was always wandering around trying to see him. I just wore my heart on my sleeve and was only happy when he was around," says Diana, who died in a car crash in Paris in 1997.
Her obvious infatuation led to palace gossip, and "It got so difficult and eventually he had to go. And, um, it was all found out and he was chucked out." Claiming that Mannakee might have been "bumped off" after their relationship came to light, she said she suspected that her husband was secretly seeing former girlfriend Camilla Parker Bowles at the time.
The tape is part of a series made in 1992 by her public speaking coach, former actor Peter Settelen, who recently sold them to NBC.
"Yeah", she said when asked whether he provided the "intimacy you weren't getting."
Charles broke the news of Barry Mannakee's death while they were in a limosuine headed for a high profile function in Cannes, she said.
"Charles told me that he was killed in a motorbike accident," she said.
"And that was the biggest blow of my life, I must say. That was a real killer. Charles thought he knew but he never, never had any proof. And he just jumped it on me like that and I wasn't able to do anything.
"I sat there all day going through this huge high-profile visit to Cannes - thousands of press. Just devastated."
In a part of the tape broadcast late last month by NBC, Diana said Prince Charles was like a lovesick puppy when he first met her at the funeral of his uncle Earl Mountbatten, who was murdered in 1979.
As soon as she sat down next to him to express her condolences, he was all over her. "And um yeah, I was flattered but I was very puzzled," she says.
After their first meeting, Charles would follow her everywhere. But at the same time was extremely erratic in his behaviour, she said. He would call her every day for weeks and then disappear for a few days without a trace.
In the latest tape, she said after Mannakee's death, "I used to have really disturbing dreams about him. He was unhappy, wherever he's gone to."
But a secret trip to the cemetery where Mannakee was taken-- "I went to put some flowers on his grave"-- revealed that the bodyguard had been cremated.
"He was just chucked over the ground. That absolutely appalled me. But there we are, I wasn't in a position to do anything about it."
But she laid her flowers anyway, and "The day I did that, the dreams stopped. It's strange, isn't it, like a sort of recognition.
"I should never have played with fire but I did. And I got very burned."