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A source reveals how Jennifer Garner is dealing with Ben Affleck's relationship with his alleged former lover Lindsay Shookus. The 45-year-old actress is reportedly not happy that her actor ex is moving on with a woman from his past.

"It's not an easy situation for Jen," the insider tells PEOPLE. "She knew it would happen soon. Ben just can't be alone." The source adds, "It would have been easier for Jen if Ben would have just dated someone that Jen wasn't familiar with."

While "of course there are issues given it was a major affair," another source says, "Jen is her own woman and is completely fine. Nothing is new for her here." She is now focusing on her children. "She's in mama-bear mode," the source claims, "putting the kids' happiness first."

Shookus reportedly hasn't met the kids. "Jen and Ben have talked about not introducing the kids to people they date until it's serious," the insider explains, adding that Garner "isn't thinking about dating - she just wants to protect and spend time with the kids."

Ben Affleck reportedly started having affair with Lindsay Shookus, a "Saturday Night Live" producer, in 2013 when he was still married to Jennifer Garner and Shookus to Kevin Miller, a fellow "SNL" producer. Affleck and Garner split in 2015 after a decade of marriage. They have three kids Violet, 11, Seraphina, 8, and Samuel, 5. Shookus has also parted ways with Miller, but her romance with Affleck didn't go public until recently after he filed for divorce in April.

Affleck has since been spotted going on dates with Shookus while Garner was recently seen putting a brave face when going solo to the gym. Despite their separation, Affleck and Garner occasionally spent time together with their kids. The exes recently celebrated the Fourth of July holiday as a family.