My best mate and ex had sex. what to do now?

Basically the girl involved in this is one i met around 3 years ago when we were young teens around 15/16. She was always the girl that never made off with anyone and we both had soft spots for eachother and liked eachother but it never really progressed from.friendship to a relationship. When.we first made out we both told eachother how we felt and she was saying how she wanted to start a relationship etc. Couple days later i find out she goes back to her later tht night with another boy and was getting off with him, she carries on like normal and loses her virginity to him. So couldnt trust her from then on but stil madly loved her. After her fling with this boy we hooked up and had sex a few times things ddnt work out as i.couldnt trust her. After this she turns into a slag and sleeps with a lot of guys including my best mate.most recently. Hes always been too close with her but knows how i feel about her. Her and i have always been so close talking to eachother every day but i cant trust her. What do i do about my best mate i cant be pissed off at him as theyre both single they can do what they want but i feel betrayed as i know he knows i stil do like her a lot and if i ddnt put the pieces together n find out what they did neither would tell me. I dont want them to do it again as i feel hurt when ti hear such things. I should forget about this girl but i cant.