Ben Affleck 'Couldn't Deal' with Jennifer Garner's Expectations: Source

A source says the actor never felt he could be "good enough" for her

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Ben Affleck and Jennifer Garner: After 10 years and three kids, what went wrong? Subscribe now for the REAL story behind the split, only in PEOPLE.

While Ben Affleck and Jennifer Garner were open about their marriage troubles and attended couples therapy for years, a source close to both says in PEOPLE’s new cover story that one reason the marriage unraveled was that Affleck had a hard time meeting Garner’s expectations of him as a husband.

“When you’re told you’re not good enough, you start to believe it,” says the source of Affleck. “Then where does that leave you as a couple?”

“It is very tough. It wears on you,” adds the source. “He couldn’t deal with her expectations. Part of why this happened is her being controlling and unhappy.”

Yet friends of Garner insist the actor didn’t understand what a successful marriage would entail. “He underestimated the work that marriage requires,” says one pal. “And once he realized it, he wasn’t interested in it.”

The couple, who announced their plan to divorce in an exclusive statement to PEOPLE on June 30, have made it clear that their children – Violet, 9, Seraphina, 6, and Samuel, 3 – are a top priority.

“It’s nice to at least try to move in a positive direction with your life,” the source says of Affleck, who will remain living on the same property as his family in Los Angeles. “It’s not happy, it’s not easy, but it’s nice to look forward and see that this all can work and keep moving forward that way.”

For much more on why Affleck and Garner split, pick up the new issue of PEOPLE, on newsstands now