Please help me to stop this?

I don't know what the hell happening to me...First time since all this depression make my live a pile of puke I felt happy, for long, stable, wrok, sports, fun. And now this...Some girl that I don't even know became friends with my bf...he was kind a colder to me since then or around that time, he gave rational reasons for this. About her he say that they are just friends. He had female friends before, but I never felt jelaus at least not that much. Everytime i remember that she exist I feel bad, feel pain. I trust him that nothing going on but some annoying little voice make me doubt. I just don't wan't to think about her...she now active in his forum that was down for a while and I refused to write there anymore even that I am admin because he made her a moderator in a few section. He say that she promised to help him with the forum invite people...I don't write there anymore, he will not remove her because he says that he hate that I try to control him, that I don't even know her