Pissed Off! What Should I Do?!?

Alright, so when me and my ex-bf David broke up for the last time, he told me that he had still LOVED me.He still can't get over me even though it's been over two months now. Now he's going out with the school slut Sadie. She probably dated 18 guys and 1 girls in the past two years! I'm mad that he lied to me! He doesn't even like her! What should I do?