Responses
Life
Here is one where they do break up: Life Senjou no Bokura. Although I wouldn't really say the break-up person was slapped in the face and the ex wasn't dating another when they reunited. It's a bittersweet one so be prepared for that if you do read it. This is one of the only mangas I remember making me cry. Although it might not be what you were looking for since it seems like you want that "ha bitch in your face kinda feels" lol but it's still a good read if you haven't read it yet. Hope this somewhat helps (●'◡'●)
Is there a manga out there where one of them wants to break up despite being mutually in love just so the other could have the "ideal happiness with the opposite sex" and unlike in this manga, they really do break up and then the one who wanted a break-up meets his ex some time later with another man and then he so regrets it?? I don't care if in the end, they still end up together or not... I just want to see a scene where a person's arrogance and self-righteousness (or really, selfishness) came back to slap them right on the face extra strong.