It bothers me when my girlfriend spends time with her ex lover.
Really? Why don't you want her doing that?
I'm afraid that she might fall back into an abusive relationship.
It's perfectly fine that you don't want her spending time with her ex. You should tell her that this bothers you.
I'm not going to tell her that, I'll find a way to break up with her so that I don't look bad.
I know it may be difficult to have this conversation with her, but your heart is in the right place and I'm sure it is best to speak with her about it.