I hate my ex girlfriend. how to deal with my anger?

You seem to be experiencing the stages of grief. Denial, pain, then intense anger, depression, and finally acceptance. You're at Intense Anger. You understand she is an idiotic slut yet your lingering feelings of betrayal are causing you to be insanely angry at her. The only way the anger will p is if you move on. It might take a couple of weeks before you completely calm down, but in the meantime, avoid going to her Facebook page (delete her if necessary), delete her off your phone, delete her off your aim/yahoo/whatever you use to chat with. Cut off all communication. That is the only way you'll get over her and your anger.