The couple announced their plans to divorce last week after 22-year-old Sara Leal claimed to have slept with Kutcher, 33, on the weekend of his sixth wedding anniversary in Moore, 49, in September.
A source told People magazine that the allegations were the straw that broke the camel's back, but that their marriage was already on the rocks because of her insecurities and his wandering eye.
'[The marriage had] been deteriorating for a long time,' said the insider. 'Ashton was ready to end it, [but] he wanted to wait for Demi to actually do it. He loved her but couldn't live with her.
'[Demi] really wanted to save the marriage but couldn't deal with the pain of his cheating.'
'They were fed up with each other,' added a second source.
'If anyone was uncomfortable with the age difference, it was Demi, not Ashton,' explained a third contact. 'She drove him crazy after a while with questions and doubts.'