How to deal with a mean stepdad?

I am seventeen, and I will be eighteen in October. I live with my mom and her husband. My dad is...not in the picture. My mother's husband is a crackhead that controls everything that I do. He makes me play my instrument everyday for half and hour, and if I dont have time...which is often.. I need to make up the time (with homework- I take AP college credit courses, and work two steady jobs, and also babysit frequently) He yells at me and threatens to beat me all the time. My mom ignores this all because it is 'not her issue'. I dont know what to do anymore. I feel like things wont ever get better...