I was the other woman...

Seven months ago I got really close to a guy. We were friends with benefits but more than friends if that makes sense. I'm bulimic and he was almost my security blanket. He made sure that I was at least keeping one meal a day down. He made sure that I always saw the good in myself. 5 months into the relationship I found out he had a girlfriend and it crushed me. I fell into a deeper depression and lost all self respect for myself. A couple of weeks ago me and his girlfriend became friends unintentionally. We realized a couple of days in we realized who each other were and decided to stay friends no matter what our past was like. The guy is still in my life, checking up on me to make sure I'm doing okay. But I know this bothers her. She's a good person and I don't want to hurt her. But I also can't let go of him. I need to do something about it before I lose both of them. But I don't know what to do. Please help me figure out what to do...