One of my good friends is now going out with my ex ?

basically, me and my boyfriend split up 3 months ago, and now he's going out with one of my good friends. I know I shouldn't let it bother me & I should be over him by now, but I'm so upset and crushed and I'm soo jelous! me and my friend keep arguing now, and to be honest I don't really want to speak to her cus I feel so betrayed! should I just forgive her and move on? any other advice?