"It was Ana who ended it," the source tells OK!, explaining that even though they'd been quietly having problems "for a while," Affleck, 48, saw a future with the Cuban actress, 32, and was even planning to propose. "He's been pretty lost without her and is taking the split hard."
The pair's troubles peaked late last year, after Affleck invited de Armas to move into his Pacific Palisades home.
"They started bickering a lot and getting on each other's nerves," spills the source. "Ben wanted to be with her 24/7 [and it made] Ana claustrophobic. She felt like the fun was gone and needed some space."
Once she got a bit of alone time, "Ana realized it suited her better and that she had taken this romance with Ben as far as it could go," the source continues. "Ben was doing all he could to change Ana's mind, but over the holidays, she told him plainly that she wanted out."
Since then, the dad-of-three has been leaning heavily on his family and close pals, including ex-wife Jennifer Garner and longtime BFF Matt Damon. (OK! previously learned that ex Garner has been there through it all. "Once again, Jennifer Garner is picking up the pieces,” a source said following the split. "Jen doesn’t want to get back with Ben, but she does love him. He doesn’t have many friends and she feels like it is her role to help.")
"Ben fell hard for Ana," shares an insider, adding that he'd hoped they could "work through their issues."
However, "The breakup isn't a huge surprise to people around Ben. He threw himself into this romance at 100 miles an hour," explains the source. "It's been tough, but Ben's a fighter. They know he'll pick himself up like he always does."