When I turned 17 everything in my life started to change and I think it is mainly because of my mother.
I got my P's so she started thinking that because I was independent I didn't need her she basically stopped talking to me, stopped buying ingredients for my lunches and she made a new friend.
I hate this friend she spends all this time with him and he is creepy and sleezy and they are texting each other all of the time and what's worse my step-dad doesn't seem to care. For example he is currently moving house so my mother isn't going to work because he needs help moving and she will drive him places for his work or buy him new jeans or whilst I'm in the middle of my end of year exams invite them over, where he gets so drunk he has to stay over and in doing so destroys my exam notes.
And what's worse as a large 17 year old girl my mother confessed to me that she would rather me be anorexic than fat. I was feeling self-concious enough after trying pills, shakes, lite and easy, going to a dietician and basically healthy eating and exercising and not getting any results she tells me that she would rather me be unhealthly skinny. So since my 17th birthday which was 6 months ago I have gained 10kg and I don't know why.
And the whole time it seems my mother couldn't care less. She has come home every night at about 6 drunk she says she went to work but when I call they always say she left at about 12 when she only arrives at 10. That means she spends 6 hours at the pub drinking.
And with all this my step-dad seems to think it is all my fault because he yells at me almost every night over nothing, today he yelled at me because I hadn't filled 3 water bottles.

Please help me I have no idea what to do with myself or my mother and its driving me mad, so much so I can't sleep anymore.

I think you have to confront your mother. Stand up for yourself. If she prefers you to be anorexic tell her you don't care. Act like brick wall when they are shouting at you because it seems eventually she will come crawling back to u