It seems to me that something is off in the connection between the two of you...2 break ups in a short period of time indicates that something just didn't feel right. Is it that you are afraid to be serious with someone, or is it that she just isn't the one with whom you share a deep physical and emotional connection? It may be that you desire to go back to her and miss her because it feels nice to be with someone, so it may be less about her when you go back and more about being with someone and having someone to do things with. I would not say put it off altogether, but maybe take things slow and date and be loose and carefree about it...then you can let things develop without the pressure of having to say you are in a serious, exclusive relationship. Clearly she likes you because she keeps getting back with you, so maybe keeping things light is the way to go until you can decide if the connection feels right.