In the end, you're the one that has to go to college. So no, it's not selfish at all to ask of her to back off. Motivation through pushing someone isn't a very good way I've learned.. Well, to some it may be. But if you feel overwhelmed due to your mother, then it's not selfish.
I haven't been to college, but I can only imagine that it's already stressful enough without outside pressures.
Unfortunately these know-it-all types don't back down easily. Stand your ground, and every time she tells you (or implies that you are) selfish or lazy, contradict her with words and actions. If you're studying hard, doing what you have to do, there should be no reason for your mother to bother you like that.