The 63-year-old actress was only 17 when she fell for John, 46, who was married to his third wife, Linda, at the time and Bo still feels regret over the affair decades later.
She told the Daily Mirror newspaper: ''That still breaks my heart. Being part of breaking up a marriage is a horrible thing and I still can't believe I was a part of it.
''When I look back on being 17 with a 46-year-old man, I thought I knew everything. It felt right at the time. I was so in love with him.
''At 17, I felt I knew everything and was ready to take on the world. To fall in love with this exciting man and live in Europe was a dream.''
John left Linda for Bo and the pair set up home in Europe as Bo was under 18, the age of consent in her home state of California.
They tied the knot in 1976 and stayed together until John's death in 1998, aged 71.
Bo - who has been in a relationship with partner John Corbett for the past 18 years - has praised Linda, 77, for how gracious she was about the break-up.
She said: ''She's such a beautiful human and I'll never forgive myself. I'll never make that mistake again - I'm forever sorry to her.''
And Linda even agreed to take part in the documentary 'Bo Derek: In My Own Words'.
She said: ''Linda's the last person I'd have expected to say yes. I was completely blown away. It brought back a lot of the sad times when I was beginning my relationship with John.
''It was a very painful time for everyone involved. I look back at that time in my life as a terrible mistake. I think she's been so extremely kind to me and I don't know why.''
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