The magazine says that Jen told friends last year that Ben admitted to cheating on her with another woman, but she "eventually decided to forgive him."
After the former couple officially announced their split after 10 years of marriage, it was revealed that they had been separated for six months. Jen, Us said, actually kicked Ben out after the supposed cheating admission.
The magazine said that mistrust had been growing for a while and that Jen was fed up with Ben's gambling and drinking, something that had been a sticking point with the couple for years.
"She tried to leave him a few times, but he would beg her to stay," a source said. And Jen did, mainly for the sake of the former couple's three children: Violet, 9, Seraphina, 6, and Samuel, 3.
Unlike his gambling, though, Ben apparently hasn't been all in in his marriage. Ben, a source told People Magazine, "told [Jen] he had not been committed to the marriage within the past few years," later adding, "Jen is a fighter. She would not give up unless there was a good reason."
An Affleck source said the "Batman v Superman: Dawn of Justice" star isn't doing well with the split. He and several of his friends, however, have vehemently denied that there was any infidelity.
"This is just as hard on him and difficult for him," the source said. "Any divorce is tough on everyone. He is leaning on his mom and close friends."
Regardless of the potential bitterness, the family still decided to take a Bahamian vacation after the couple's announcement.
"She always fought against the urge to throw her hands up in the air and walk away," an insider said. "They've been on the brink of splitting up many times before, but it's Jennifer's worst nightmare for their children. Her biggest desire has always been to keep the family together."
According to People magazine, few of the former couple's friends were surprised to hear the news. "They have both been very open about their marriage struggles," a source told the magazine. The split, another pal said, "was inevitable. If there's any surprise, it's how long it took to happen."
People even quotes a source who said Ben couldn't deal with Jen's view of him.
"Saying he's a bad father and a husband is an easy story, and everyone has wanted to clock him as a womanizer and gambler," says a source close to both. "He couldn't deal with her expectations. When you're told you're not good enough, you start to believe it."
In the end, though, Jen held on as long as she could.
"She loved Ben," says another close friend. "But there comes a time when you have to say enough is enough and take care of yourself."