Family problems, need an answer please ?

Well, im a 15 year old boy, all my life i have had to deal with my mom. She's the most disrespectful, annoying person in my life. My father left me and her when i was 7, since then, my current step dad has supported us. I love my step-father to death, and all she does is tell me and him how worthless we are, even though she does even less. My mother has a naturally yelling voice, that just pisses me off. She thinks she knows her own son, but she doe's not know me at all. I've NEVER talked to her about any of my personal problems because she thinks she's always right, she always yells at me, and there is no communication between us at all. I just cant talk to her, i don't know how else to explain it. EVERYDAY there is screaming at my house (no exaggeration) over stupid stuff. My mother does not trust me at all, and she has no reason to, because i've never done anything to lose her trust. I cant live under this household with trust issues, because then i wont be able to do ****! I cant stand this lifestyle. I am literally grounded every other month, for a month, for being disrespectful because she never lets me explain myself, so i just end up cursing her out. And i know its wrong to be disrespectful, but its nothing compared to what she does to me. To her, i have no privacy. She always goes thru my phone and my myspace. I know i'm 15 but can i have SOME privacy ? It just pisses me off. What the ****, just while typing this message she comes into my room telling me that i HAVE to eat. Im not hungry. I dont HAVE to do anything! Its just the little things she does. She cannot dictate my life...Now, just yesterday, she was yelling and screaming at my step-father. I got into it and said "stop screaming for once please!" 5 minutes later after she's done arguing with my step dad she comes up to me during dinner and says "when im arguing with your dad, dont get into it" i said" i can get into whatever i want just please stop yelling its annoying" soon after all that she said "you know what, im gonna talk to your dad so you can move in with him, so i can get you out of my life already." My biological father lives in Miami, i reside in Tampa. But what she said really hurt me. But what she said next hurt more: "And when you get to Miami, dont even bother calling here because i dont care whether you're dead or alive." Its the worst feeling hearing this from your own mom. I've been living a constant struggle in communication with my mom. I honestly think i hate her. All she does is tells me how worthless i am. What do i do wrong ? I have a 3.5 GPA in high school, I help around the house sometimes, im not a pain in the ***. I never get into trouble in or out of school. I help my little sister with her homework. I tutor some kids. I wanna make something out of my life. And all she does is tell me how worthless i am. I swear i would be poor, live in a dumpster, still be in Cuba, or anything bad. If i can just have the totally opposite mother i have now. I'm a guy and i cry every week because of all the hurtful things she says. What do i do ? :(