It was an unlikely romance from the start – and in the end, the six-year marriage of Ashton Kutcher and Demi Moore fell apart because of issues that had long fueled skepticism about their relationship: their famous 15-year age difference, his wandering eye, her insecurities.

As a result, the marriage “[had] been deteriorating for a long time,” a friend of Kutcher, 33, tells PEOPLE in this week’s cover story. “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. “

Moore, 49, “really wanted to save the marriage but couldn’t deal with the pain of his cheating,” adds the friend.

“They were fed up with each other,” a second source tells PEOPLE.

Several friends note that the age gap between the former power couple – who are worth an estimated $ 290 million – caused Moore to doubt herself.

“If anyone was uncomfortable with the age difference, it was Demi, not Ashton,” says a Kutcher pal. “She drove him crazy after a while with questions and doubts.”

For much more on Demi and Ashton’s tattered marriage, pick up this week’s issue of PEOPLE, on newsstands now

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