No one likes a cheater, but I know exactly the situation you're in right now.
I was with my ex for 2 years. Things were falling apart for the last 6 months, I went to uni (the same one as him), I got ridiculously drunk one night and kissed someone else. Through my jagerbomb induced blur an alarm bell went off, I pulled away and somehow ended up back in my halls devastated at what I'd done, guilty as sin itself, but the feeling underneath it all was that I didn't actually regret doing it. I assume that's sort of the same thing that's happened to you.
I don't feel good about what I did. Every time I think of it I have a surge of sadness and guilt and right now is no exception. There's a difference between guilt and regret though- I felt guilty because I didn't want to hurt my boyfriend, but the part about not regretting it came from realising that as much as we wanted to hang onto it our relationship had evolved into something neither of us recognised, and essentially we were still together because it had been so long that we couldn't imagine life without each other. I finished the relationship, confessed what I'd done, and he was wrecked. It ripped me to shreds seeing him like that and I hated myself for a long time. I met up with him yesterday. He's fine, new girlfriend, life sorted out. So that put my mind at rest. Anyway, I digress.
I don't know how many parallels you can draw between my experience and your own, but you need to think really carefully about your next move. There's a few questions I asked myself that may help you.
Are things reaching their natural end with your girlfriend, or are you just looking around and thinking that everything else seems much more fun than what you're used to? Is there anything deeper with this other girl or was it more of a matter of convenience? If you dump your girlfriend and go for this other girl only to find that she doesn't want to break up with her boyfriend, what will you do then? Are you actually ready to jump from one long term relationship straight into another with someone you don't know that well?
Work out how you feel, then work out what you want and talk to this other girl about how she feels and what she wants too. If you do break up with your girlfriend, you need to be honest with her but remember that this is your fault, not hers. I don't want to assume that you're the kind of guy who would subtly blame their partner for their cheating, but if you do get tempted- don't.