Ok, here's the deal. No one on the BOS forum can help you get through this problem. You need to sort it out for yourself. That means weighing up both your gf and this other girl. Which one do you have more feelings for? Is this love/lust you feel for them?
This girl has said no to you before. To me, that's a pretty clear message.
If I were in that situation, I'd tell my girlfriend I wanted a break, and then I'd think about it more. If your girlfriend tries to get back in touch with you during this 'break', she obviously wants to be with you, so you should think about how much she could love you. If not, it's clear she's not worth your time, so I would break it off completely. Then, sort things out with this other girl. Tell her that you're confused, and tell her how you feel about her. If you get a negative reaction, she's not worth it, so FORGET her. You'll never get anywhere if you keep her on your mind when you know she doesn't feel the same about you.
If love is done right, it shouldn't suck. So maybe you should question what you call love.
Good luck with this, it sounds tough, and you sound like a decent guy who has thought the feelings of everyone over, not just your own.
Just don't let it take over your life, whatever happens, and remember that there are other things to look forward to.