Man has an affair and confesses to his girlfriend thinking it will fix their relationship and she dumps him

I honestly can't remember if this was the whole movie or just an aspect of it. I can't remember the genre either, honestly (probably romance/drama?)

I just remember that a man is having difficulty with his relationship and he ends up sleeping with another woman, after which he realizes he really does love his girlfriend and wants it to work out. He goes to her optimistically to confess he's cheated and that it's opened his eyes. He expects her to be ok with it, but she very calmly tells him that she can't be with him because of it.

Anyone happen to know?