Consider your biggest goals in life and be honest with yourself if they are negotiable. If your girlfriend's goals are different and also non-negotiable, it's time to move on. Even if you are physically attracted to each other, you never fight, you get a long on many levels, etc., it will not work in the long run if your goals are different.
One of my friends has been dating a woman for almost three years. They are both mid 20s. They claim to have never fought over anything major in the three year relationship. They get along perfect... like peanutbutter and jelly... like toast and butter..
BUT.. she wants to be married and start a family in the next five years. HE, on the other hand, won't be ready for a family for at least the next 15-20. Neither is willing to negotiate on THIS one issue. They broke up, but are still very good friends.
It doesn't matter HOW hot your girlfriend is, don't let the fear of not being able to find another girlfriend as beautiful hold you back from breaking up if that's what you need to do. I notice a lot of guys in bad relationships hold on because they are afraid of being alone. You won't be alone for long.
At the end, you really KNOW deep inside whether it's right. It's just always night to get your thoughts validated by friends or TFP.