I'm looking for some manga were the main character was in love with another and found out that the one they loved was cheating on them. The cheater will continuously try and win back the affections of the one they wronged, but the protag has moved on and is trying to find a new love.

I'm not a fan of yaoi or shounen ai, so please avoid them. Also no Hentai as such a scenario in Hentai is terribly easy to find anyways.

Kimi wa pet

How does Kimi wa Pet fit?

Futago
It's not "established" yet that the main character's had someone cheating on her, but it's implied. It think it will come out in later chapters.

________________
Go to work, send your kids to school;
Follow fashion, act normal;
Walk on the pavement, watch T.V.;
Save for retirement, obey the law.
Repeat after me: I am free.

Skip Beat!. But you've probably read that!

Not yet, but it's in my reading list so I do plan to get around to it.

Does this have to be a primary aspect of the plot? If not, I'd say Monster somewhat fits.

I doesn't have to be central, but it should be something that affects the way the protagonist interacts with people, especially potential love interests.