I can't stand up to my mother?

Start by letting her know that you've tried your best, or tell her how sorry you were at what you are about to say. I think most of all be very serious and concerned, at the same time sorry and innocent. As long as you've tried your best and is willing to try harder, your mother shouldn't take it so harsh. She is your mother after all, so whatever she says is only for the best. She loves you, remember. Good Luck ^^