Unfair Treatment From My Mother?

I'm not sure why she treats me so differently than my other siblings but she does. I am now 18 but this has happened my entire life. I'm her only child that doesn't have another parent or grandparents. I have 4 other siblings. My older sister got pregnant at the age of 16 and moved out but her grandparents raised and paid for her when she was a kid. My younger sister lives with her father and is a spoiled brat. My younger twin brothers live with their father too. My grandparents use to be around a lot but they both died when I was very young. My father has rarely ever been around pops in and out of my life as he pleases. My point is that I have always been here for my mother and supported her through everything: all the loser boyfriends she brought home, all the physical and emotional abuse she has put me through, all the drugs she use to do. Everything I never abandoned her. She has always called me her middle aged child because I have always been mature for my age and took on responsibilities I never should have at that age. Though no matter what I do I'm never good enough when my other siblings are around. Anything they ask she gives to them. All my siblings have iPhones and I have a crappy prepaid phone I have been paying for since I was 15 all by myself. She buys my siblings whatever they ask for but I ask for something simple and I'm asking too much. I don't know why she treats me like this when I have been supporting myself since I was 16. I talk about moving out and she practically begs me to stay. I don't want to stay around and be treated like crap all the time while my siblings are the golden children and I'm treated as the black sheep of the family. She needs to learn that she treats me unfairly. What should I do?