I hate my stepmom. please help?

Ok for a little info on her, she has been around since I was 2 ( now 16). She has always been the devil that lives at home with me and I hate her. She is only nice to me if there are people around or I have been gone a while. Even if I've been gone a while she is only nice for a day or two. When I was little she would go down to my room when no one else was there and yell and hit me and when she would walk out if the room she called me stupid. My dad doesn't know about any if this but I'm getting sick and tired of living with her and even though I treat her like she is the nicest person I know, she still treats me like carp. I recently just moved away from my biological moms house because my dad thought I would get better grades. Now after 3 months being here she is driving me insane and I dont want to live here anymore. She treats my half siblings like they are perfect even though they treat her like crap. I don't know what to do anymore. I'm about to just snap.