My best friend is having with my ex. helllp!!?

Okay let me give a little background info first. So my best friend I've known him for at least 5 years and we have always had fun. Now my ex, we went out for about three months. We both came to the conclusion to break up. We're still good friends though like we hang out and go to the movies even after the break-up. Okay so about a few weeks after we broke up my best friends started hanging out with her very frequently. I was bothered by it so I went to him and specifically asked him not to hang out with her too often because even though we are good friends its still awkward and not to try anything with her. He said oh yeah dude no problem. They continued hanging and it still bothered me but since we had that talk I though it was just friendly hanging out, you know? I hear from my other best friend that about a month later that he told him that he has actually been having with her for a while now. Enraged with fury I couldn't even see him that day. I couldn't concentrate in cl I couldn't do anything! The only thing that helped is football. It helped me release some anger but I guess nothing really helps get rid of it all. Everybody is telling me to fight him but I just don't know. I really wish I could go back to being cool but he really crossed the line. I really don't know what to do please help!