You have two issues: facing the fact that your current girlfriend is not who you want, plus liking the other girl.
As far as the first one goes, you can't stay with someone just because you feel bad for them. While it's very nice of you not to want to hurt anybody, you'd be better off, and she would be better off, if you just let her down gently now, before the relationship goes on longer. The sooner you end it with her, the sooner she can feel better and move on.
As for your second issue, consider the fact that your attraction to this other girl could be just a case of "the grass is always greener in the neighbor's backyard." If that's not the case, and you really have things in common, etc, then go for it! Why shouldn't you be happy?
I hope this helps - keep us posted!