I don't know if I can really help, but here goes.
I met this girl in 11th grade physics. That was 3 years ago. She had a boyfriend then and I didn't feel any pressing motivation to ask her out or anything at the time. Over then next year I saw her occasionally and I wanted to be with her - however, I lost my balls and never said anything. Then one nite about a year ago my neighbor shows up with her. They were just chilling out and came over and we all talked for about an hour. She was still going out with the same guy, however they seemed to be drifting away from oneanother. Over the next few weeks my neighbor and her continued to drop by and it compounded my feelings for her 10x over what I thought I had.
Well, I couldn't take it and I told her how I felt in an email. She told me she was still happy with her current boyfriend and that I was "a good friend". Well I continued to write her as she went off to college and got to know her more and all that good stuff. Then, she had a messy breakup with him early this year. I had been at my new job for a few months and I had really changed from when she last saw me and we picked up the email talk again - but she still wanted to be friends. That was what I expected. Well, she came home for the summer. I traded a few emails with her and she asked to hang out with me and the guys one nite. So we did. 3 days later she asked ME out. I didn't know what to say.
When she first told me she had a boyfriend, I was sickened. I felt like you did (not to so great an extent). I tried and tried to get her out of my head and I couldn't. Nothing worked. After about 6 months of not seeing her and not really talking to her I had begun to shake her out of my mind, but that still hurt. It just takes time. Its not going to be easy, but no girl is ever worth taking your own life for. I had that feeling at one point also. I barely ate. I walked around all depressed. Just keep in mind that it will get better.
As for me, I've been dating her for 2 months now. I'm a lucky bastard and I'm still wondering if I prayed in the right direction, did something right in the last life, bowed to the god, whatever. All I can say is that time makes everything better and, but its the wait that makes it all worthwile.
Play some CS, Q3A, whatever, just know that it isn't the end of the world. I wish you the best 🙂