Oh man. I was bawling in my counselor's office on Wednesday when she told me to just tell my mom no. I so feel for you and get where you're coming from. I've been making up excuses the last two weeks as to why my mom can't have her way, cowardly but effective.
I agree with everything Firesprite has written. I also think you should put your foot down and I know how scary that can be. It's not simply saying no but the backlash that follows, BUT you have to start somewhere, right?
I'm saying a prayer for you and sending you a big hug!