My ex had sex with my best friend the day we broke up?

My best friend had sex with my boyfriend,(stacey) the day we broke up, and my other friend kristy, knew about it the hole time and didnt tell me for a week, and now stacey and kristy are talking crap about to my ex, saying i cheated on him and that i am a whore, and now kristy and stacey are best friends with him, They want to be friends with me again, what do i do????