Ben Affleck and Shauna Sexton split

ET has learned that Ben Affleck and Shauna Sexton have broken up.

“It was casual and not serious," a source tells ET. "Ben’s only priority right now is his sobriety and his family.”

Affleck, 46, and Sexton, 22, were first spotted together in August during a Malibu dinner date. Since then, Sexton has been snapped visiting the actor both at his Pacific Palisades home and while he was in rehab at Malibu.

She recently Instagrammed a trip the two took over the weekend together in Montana.

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A source told ET last month that Affleck's relationship with Sexton has "been challenging for everyone who is trying to help him through this process."

"He has been told to hold off on a relationship... but isn’t heeding that advice," the source said. "His closest friends hoped he was just dating her and would break it off after going to rehab, but he seems to get closer to her every day," the source said at the time. "The more time Ben seems to spend with Shauna, the more he seems to want to go home."

The source continued, "Shauna has been a strong support for him by attending his family sessions and listening to the advice, but she doesn't seem willing to give him the space he needs to stay in rehab."

Affleck -- who recently finalized his divorce from Jennifer Garner -- completed his stay in rehab earlier this month and broke his silence with a heartfelt Instagram post.

"This week I completed a forty day stay at a treatment center for alcohol addiction and remain in outpatient care," he wrote. "The support I have received from my family, colleagues and fans means more to me than I can say. It’s given me the strength and support to speak about my illness with others."

"Battling any addiction is a lifelong and difficult struggle," he continued. "Because of that one is never really in or out of treatment. It is a lifelong commitment. I am fighting for myself and my family. So many people have reached out on social media and spoke about their own journeys with addiction. To those people I want to say thank you. Your strength and courage is inspiring and is supporting me in ways I didn’t think was possible. It helps to know I am not alone. As I’ve had to remind myself, if you have a problem, getting help is a sign of wisdom and strength, not weakness or failure. With acceptance and humility, I continue to avail myself with the help of so many people and I am grateful to all those who are there for me. I hope down the road I can offer an example to others who are struggling."

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