I don't know what to do anymore...

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I really have no clue what to do anymore. I feel dead inside.
This all started about 6 weeks ago. I was talking to this girl that I've known for quite a while now, she's beautiful and I felt a connection before but we decided to leave it as friends, which I was happy with. That was about 5 months ago. When we talked 6 weeks ago, I found out she had a boyfriend and that they'd been seeing each other since around the time we decided to leave it as friends. When I found out, I felt so sick I thought I was about to throw up. And I don't even know why.

Ever since then, I can't get over her. I think about her all the time. I get jealous when I think about her and her bf. I know we don't suit each other and I'd much prefer to be friends but I just can't stop thinking about it and it's interfering with my life. I've tried loads of ways to try and forget about it, I've tried focusing on her bad aspects, tried cutting her out etc but it just won't work. I would try replacing these feelings by finding someone else but like many people here I'm not a very social person at all and it's very rare that I meet another girl that I can get close enough to.
I have many other problems too, many I don't wish to discuss here. If I could just get this one off my back it would be a lot easier.

tl;dr a few weeks ago i found out that a girl I've been talking to has a boyfriend. I had feelings for her a while back which were resolved and I left it as just friends, but as soon as I found this out I can't stop thinking about her and it is starting to ruin my life.