Why is my mom so abusive and mean towards me?

I'm 17 but I still have one year of high school left (my birthday barely passed the cutoff), and my mom-who has always been physically and verbally abusive-has become a hell of a whole lot meaner lately. At first, I thought it was her job-she's loud at home but at work she takes crap from everyone-so when she began to work at home, I thought she would act a little more civilized. Now she comes into my room early in the morning calling me terrible words and hitting me with what is near. Before, she was extremely physically abusive-she used to grab my hair and literally drag me through the house on my back or knees with it, throw things at me (once she hit my ankle with a pot and now it makes permanent cracking noises) and she grabs me by the ears and arms and twists them really hard, to the point I cry out in pain. I have scars from when she cut my arm with a straightener plug-I was 12 and didn't know the alarm code when it goes off-and when she has hit me other times. Now she's more verbal-she calls me an idiot, a whore- which I don't understand since she knows I'm still a virgin-, a b****, a retard, a loser, a lazy piece of s***, unpatriotic-since I have opposing political viewpoints-, a b******, etc. I try not to get in her way but whenever my siblings do something wrong, she won't punish them-she'll just get mad at me. I have straight a's, a high GPA, good friends, I've never done anything "morally" wrong, I go to church, and-I admit that I sometimes don't wash my dish-I try to cook a meal for my family because she blatantly refuses to do so. My sister-whom I sometimes go for advice-usually starts a fight with her and the fights last for a good two to three weeks. I try to get out of the house but my mom won't let me because she thinks I'm going to get my high and have sex (which is what my sister admitted to doing on her daily walks at my age) and she won't let me get a job, even over summer, because she thinks I'll spend the money for drugs. What should I do?:(