well first of all, u got to ask yourself which one means more to u, the one you've been with and love, or the one you like, cuz its two different things. Okay from my point of view, heres what i would do first, talk to your gf, and tell her u think ya'lls relationship is drifting away. and ask her how shes feeling about it before you go one to the next girl. Because cheeting is really bad. And if you truly love your gf, you'll talk to her about how your feeling inside. I went throught the same thing, and i was always getting myself deppressed, and finally i told her how things where starting to make me feel about our relationship. and she agreed that it was starting to fall apart, Simply cuz of schooling and all the homeowrk, we never really got to spend much time around eachother like we used to. So my advise to u, would b to try and talk it out with her, see what she says on her end, and then decide if its best to breakup or to try and fill in the gaps.

Good advice up there. Also, you have to remember that even though you may care about your girlfriend a lot, you're young and you have plenty of time in the future to be settled down to one person, so don't let yourself feel guilty for wanting to play the field.