According to Demi when Ashton, now 41, was filming sexy movie scenes with Jennifer Jason Leigh, he told his wife that the actress had requested she didn't come to the set.
However, she later discovered the truth was quite the opposite.
“The bottom line was that she wasn’t worried about me being on set, he was,” she wrote. “He wasn’t honest.”
The mum-of-three daughters, who she shares with Bruce Willis, also confessed that she agreed to a threesome with Ashton for all the wrong reasons.
“I wanted to show him how great and fun I could be,” she wrote, but said their two trysts with a third partner was a “mistake.”
By 2010, it seemed the writing was on the wall for their relationship when Demi says she saw reports that Ashton had cheated on her with a 21-year-old woman when she was out of town.
The final straw came in 2011, when their marriage was on the rocks, but Demi says they were still together, and she was alerted to further cheating allegations against Ashton.
He had allegedly cheated with a much younger woman while at a bachelor party on their sixth anniversary weekend.
“I felt sick to my stomach,” Demi wrote. “I knew she wasn’t lying.”
At the time, Us Weekly first reported that Ashton had hooked up with then 22-year-old Sara Leal, who spilled all in a shocking interview.
By November 2011, Ashton had moved out of their home for good.
He has since married Mila Kunis in 2015, and the couple share two children.
Demi has remained single since her marriage to Ashton.