I've been dating this girl since March 09. We weren't official until September but the months in between we pretty much lived together.

She has shown you who she is. Do you really want to be with someone like that? What if you had a child with someone like that? Best stay away from her. You are just her safety net when nothing "better" is around. Find someone who will love you for who you are. You can't make anyone change and there's nothing lonelier than being with the wrong person.

I had something very similar happen to me once with a chick. She may have some feelings for you, but she doesn't respect you. Nothing good will come out of taking her back. She will drop you again the first moment she thinks she has something better.

My guess is you still do have some feelings if you are even thinking about giving her another chance. You were a safety net to her and now that you are gone she misses having one. She maybe does have legit feelings for you but her feelings for the ex were much stronger to continuously go back to him again and again while ignore you and your feelings. You forgave so much that it's become easy for her to do whatever she wants with you because she knows you will always take her back. Maybe she really does want to give your relationship and honest go but I want to know why you would even want to? This might be painful to hear but I have a feeling she only wants you back because you boost her ego and once she has your attention again she will feel better and be back to the same old crap. Do both of you a favor and let her go... you deserve more and I think it would be hard for you to forget the past. And as for her... she will realize that she cannot treat people that way and get away with it. Move on... I think you will find yourself MUCH if you do.

Maybe, but why would you want to?