Is it wrong to go back to a girl after she left me for her ex?

I'm in love with this girl, but she left me for her ex more than once. The problem is that she has done this before and she never knows what she wants and she is constantly going back and forth between him and me. I've told her to never call or text me anymore, but she still does. Then she came over and we slept together. She realized she is not happy with him, so she wants to try again with me. Is it unwise to accept her back again? Is it ok to justify her behavior on the grounds that she was immature but is in the process of growing up?