Nearly two years after Jennifer Garner and Ben Affleck announced their separation, she’s finally filed for divorce and In Touch has exclusively learned the reason why: He left rehab too soon.

The rehab stint had been a last-ditch effort to save his marriage, according to an insider, which obviously didn’t work. “She’s telling friends she believes he left rehab too soon,” the insider says of Ben, who reportedly spent just two weeks there.

MORE: Find Out Why Lala Anthony Left Carmelo After Seven Years of Marriage

Most inpatient alcohol treatment programs range from 28 days to three months. And though Ben claimed in a March 14 Facebook post that rehab was just “the first of many steps being taken towards a positive recovery,” Jen believes two weeks was simply not enough time, says the insider.

jennifer garner ben affleck getty images

“But he refused to stay in rehab,” the insider alleges, “saying he missed the family and would continue outpatient treatment. Jen believes he was coming up with excuses.”

MORE: Find Out How Kylie Jenner Is Coping With Her Split From Tyga

“It’s extremely distressing to her, so she decided to move forward with the divorce. Ben has never believed that Jen would pull the trigger, but the time has come for him to accept responsibility for his actions. She wanted to end things for good.”

For more on this, pick up the latest issue of In Touch — and for more exclusive content sign up for our newsletter!