When will we quit punishing ourselves?

I have a friend who just told me her husband was having an affair. This is a couple who have been married a long time. Not a short term commitment here. There are years of shared memories and children. I've been lucky somehow that with my involvement with men that I have never had one cheat on me. She told me when she found out she just went crazy. I can't imagine the range of emotions you must feel when someone has betrayed you in such an awful way. She went on to say that things got really dangerous to the point of her trying to hurt herself. I wonder why we do this? When someone hurts us why do we try to inflict pain even more pain on ourselves? Isn't what they have done pain enough? When you are rejected it rocks you to the very core of your being. You question everything about yourself. What have I done wrong? What is it about me that makes this other person feel this way? What did I do? Why do I have to feel bad? Aren't they the ones that have lost out? I myself have just come out of a relationship crash if you will. The whole experience has made me do the same kind of questioning. What is it about me? Is there some tragic flaw that just invites this type of rejection? Yet as I ponder my situation which is nothing even close to my friends I still wonder the same thing...When will we quit punishing ourselves?