I am my mother's verbal punching bag - help?

You might try reading a book called "Stop Walking on Eggshells" to see if it describes your mother. It's for those who are family member of someone with a personality disorder. You mother seems to have some of the traits -- alternating idolizing and devaluing you and irrational rages being two big ones. Even if she hasn't got a personality disorder, the book can help you learn how to protect yourself.