So many problems at home and i need advice!?

My "dad" is very abusive and treats me so horrible and i cant stand it anymore! I say "dad" because hes my adoptive dad and doesn't even treat me like a person. Here's the story: ever since i was around 6/7 hes been really abusive. He hit, kicked, stabbed a few times, ect. He would decide at random occasions that i didn't deserve dinner for IDK what reason and i would end up having to hide food so i wouldn't be hungry. since ive gotten older its gotten worse and worse. I fight back now and that just makes it so much worse. Hes kicked me out and ive had to sleep outside( which at the high altitude i live at makes for freezing nights)Ive also became his slave and i end up doing a lot of stuff around the house while he just sits on the couch yelling about how useless i am, how i should have never ben born, how my real parents must have ben drugys cuz of how f*cked up i am. Last year it got so bad that i had to go to the hospital. after they stitched me up i told them everything and of course he tells them im a nut and i ended up in the mental hospital for 5 days for "stabbing myself". Ive told the police, teachers, everyone that can help and he always gets out of it! Since i was around 10 ive cut and have overdosed a few times to try to end it but it never worked. Around the middle of last year a good friend helped me stop and my "dad" has stopped most of the physical stuff but has gotten very neglectful and verbally abusive. Im scared that he will loose it eventually and i might be hurt worse than the last. I really want to end it all but I always remember that once im 18 I can get out. But what should i do now? No one will listen but my best friend who is also my only true friend.Please help!!!