I can't advise you on breaking up because of this, but from my experience, it's not going to get better. I see it in my wife, and even more so, in my sister. In my sister's case, I feel that it's a learned behavior, because my late mother did this too. My BIL and I have had many discussions about this and he's coping, but I have a feeling that someday, my sister is going to push him over the edge.

I've been there... got dumped the same day she told me that she was engaged to the other guy (that I knew nothing about). The engagement didn't last... she cheated on him too.

So, she's telling you that she doesn't want a relationship with him and he's not a threat... where have I heard that before? Maybe it was in the r/survivinginfidelity sub. I suspect that you'll be posting your story there soon.