Oh icd, I really feel for you. The advice given here is brilliant, and I have nothing else to add to it really. I just wanted to say that I think you are incredibly strong to have coped with losing your dad, and to manage your mum's behaviour so it doesn't affect your education and so on. But I think you definitely need to come out of holding your silence, and tell someone you trust, who can be objective about it.
If you need to talk, or just let it all out, feel free to PM me on SportsBank and I will pass on my MSN/email, if you feel this is appropriate. I don't want to sound dodgy or anything though; I know you are 15! :S Anyway, I will dig myself out of my hole and leave it at that!
Lots of luck hun x