Bergin, 35, details his long-running relationship with Bessette in his book "The Other Man: A Love Story: John F. Kennedy Jr., Carolyn Bessette & Me," Friday's New York Post reported.
Bergin and Bessette became lovers several years before Bessette married Kennedy when they met at Calvin Klein, where Bergin was a top underwear model and Bessette was a publicist.
Less than a year after Bessette wed Kennedy, Bergin said they renewed their affair, with Bessette telling Bergin she suspected her husband was having his own affairs.
Bergin and Bessette continued seeing one another until early 1998 when Bergin decided to end the romance.
Kennedy, Bessette and Bessette's sister, Lauren, were killed July 16, 1999, when their plane crashed off the coast of Martha's Vineyard, Mass.