Have you only been treating her like an arse while she's been in communication with this other guy? If so, it is somewhat understandable. The green monster named Jealousy can be tough.
If you treated her badly before, I understand why she is looking elsewhere.
She may be confused, but that doesn't mean she gets to dictate the relationship. The ball is in her court, and you have given it to her. Take the ball back and decide where to go from here. Your life will be less stressful if you take charge. If you want to stay with her, tell her that you don't feel comfortable with this other guy hanging around. If she respects you, she will honor your wishes (not commands!). If she is still confused, let her go. It's tough to do, but you may have to.
Respect yourself, don't let someone else run your life. A relationship is about sharing and mutual respect.