A person can only be hurt so many times before they shut down in an effort to protect themselves from more hurt. You may have been in a type of emotional shock at first when you were trying to fix everything--as if you were on automatic pilot. At some point, it must have become obvious that if it were easy to fix, it wouldn't still be happening.
If your husband is sincere about wanting to stop his infidelity, he should show you that he's serious by going to therapy himself and also going to Sexaholics Anonymous meetings to learn how to control his behavior so he doesn't keep hurting you over and over.
You can find support for your own pain by going to S-Anon, http://www.sanon.org/sanon.htm for the spouses of people with sex addiction. You'll get a lot of support from women who are going/have gone through the same hurt and confusion that you're feeling.
Go easy on yourself--it is normal to have a wide range of responses when you find out your husband has been cheating. To read more about sex addiction, read books by Dr. Patrick Carnes--he does a great job of explaining it. Here is a link to his website: http://www.sexhelp.com/