You can have a chance with her if she is willing to give you that chance.
From what you've described, the situation does not seem very optimistic. She disregarded your wish not to date others, she moved out and is with him, now she's unhappy for whatever reason and wants to come back to. But, you have to make sure that if she is to come back she is doing this to be with you and not just to avoid him out of convenience.
Her behavior appears immature and opportunistic to an objective observer. Even if she is not certain about what she really wants, there are different ways to handle that without putting you in the middle. While the two of you are apart, use the time to figure out what you want. Even if you love her, can you trust her that this won't happen again down the road, and how is she going to prove that to you?