Help...im Lost And Confused

hey world, i was at uni enjoying my time like you would and i started speaking to this one girl. she was single at the time and we were just really close and we really liked each other and i couldnt stop thinking about her. she decided to get back with her ex bf and we still talked like normal and carried on talking about everything, it got to the point where we talk like we were in a relationship and talking about sex every day. i met up with her after uni had broke up for the summer and we got really close and nearly got together which both of us wanted but she told me we cant. she text me saying all you want is sex from me. a couple days leter i told her i was in love with her and wanted to be with her. things were going fine for a week or so as we talked all day and all night then i went on holiday and as soon as i got back she realised she was being a **** gf and wanted to break all ties with me. i snapped and told her this is all about sex nothing else so lets do it and you can **** off. a couple days later we started talking again as mates but we havent got back to how we were and talking fine again as she wants to have a relationship with her bf.

whats killing me is the fact i cant get her out of my head! all day and night i think off her with her bf and not me. at work thinking about whats shes doing and if i should talk to her. why it couldnt of been me and if it will ever work out if im there for her now as a mate. i hate the mind ****! as i cant get over it and stop thinking about her at all. just depresssed and upset all day long as i really wanted to this work. its not fair falling in love for someone and having someone to call your best friend to talk to and they disappear like they dont even care to someone else and being left alone...