My friend zone pain (please read!)

I've been very close mates with a girl for the past 2 years. After going to an all-boys school and sheltering myself from seeing girls, she was the first I ever got to know at uni. For the first few months we had a great friendship- spent lots of time together and there was never a time I had a relationship on my mind. As time went on, my feelings for her increased and we continued doing even more stuff together. Then I finally went for it and asked her out but she wasn't interested. In my mind I knew I wasn't good enough for her but it was worth trying. I went back to being friends with her and we lived together in a house for 2nd year. This soon became agony for me. Hearing her bring guys back to our house, even seeing her talk to other guys made me jealous. She had 3 boyfriends that year and each time the relationship ended, she'd go back to spending time with me. I've tried to cut contact hundreds of times because I know it's not healthy for me to keep on doing this. It's all I think about before I go to sleep and I'm not remotely interested in other girls. Every time I cut myself off though, I end up going back to her because I can't leave her. It's as though I don't want this idea to go....almost like being a sado-masochist :( Any advice? I'm killing myself over this and it's not even her fault. We went out to dinner a few weeks ago and apparently one of my other mates has just asked her out. She said she wouldn't date him if I wasn't comfortable with the idea. The idea of them going out makes me cringe, but I don't want to stop her. Equally I don't want to keep torturing myself and need to move on with my life. Any advice? :(

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