I'm So Jealous I Want to Cut Myself

I have been best friends with J for eight years. But she's always been the pretty one, the skinnier one and the one with bigger boobs. 

I had a boyfriend for 2 years and when he broke up with me, he chased her, and even though she said she didn't want him at all, lately when they met up she gave him a big hug and I was so jealous.

Then one night I slept with this guy, M, who was a housemate of a friend and I really liked him. The next night I invited J to come stay the night and party with us. She flirted with him the entire night. They spooned on the couch and had a tickle fight for hours, in front of me. I wanted to die. I found out later they slept together that night and I felt so betrayed. My friend has a boyfriend and I had to console her about cheating on him, then I thought that she wouldn't talk to M ever again.

Then today I hear that tomorrow they are going out for lunch. Just a friendly lunch, of course. Just like what they were doing on the couch was friendly.

I am sick of guys I like liking her better. I am sick of my friend flirting with guys and then telling me I'm being stupid when I talk to her about it. It's so obvious that she's flirting but she just won't admit it because she doesn't want to think she is the type of person who would flirt with someone while they have a boyfriend.

I really feel like cutting myself. And I'm not just being dramatic, I have before, not for this reason but right now I am so consumed with jealously that I don't know what to do.