Why do I still want to be back with her?

After almost 4 years together, she broke up with me and started talking to a guy not even a week after. I have been deeply hurt and saddened but why is it that there is still a feeling inside me that would still want to be back with her if she came calling? A part of me says no way I should get back with her after what has happened but a part of me says that I can forgive her. Can anyone help me understand these feelings a little better and tell me what you think I should do?