My life is falling apart, I need help, please!?

I recently lost my nan to cancer in January. It literally broke my heart, an my family hasnt been the same since. We are all not talking to my grandad anymore, because he was being a **** to my nan, and he was stealing from her even though he had plenty. Its like me, my sister and my mum are in the middle. My 2 aunties arent talking to my other aunty and uncle, they just dont get along, so we have to go on seperate days out together. I am so stressed over exams, im doing my GCSE's. Im only 15, and I have to look after my mum when she gets that depressed that shes constantly crying. She has turned to drugs to help her sleep, and so has my sister. I have to sit there whilst they smoke it in front of me, and it breaks my heart. We are low for money as it is, and just everything is going wrong. I wish that my nan was back, because everything was fine then. I havent seen my dad for 13 years, and I recently found him on facebook, I told my sister this, as we had a heart to heart talk, and she told me some of the stuff he has done (not to be repeated here) and I was sick to my stomach, but I still wanna talk to him :/ I tried adding him on facebook, but no reply. My friends are acting funny with me, and im constantly in an argument with at least one person, I snap at my bestest friend because im under so much pressure, and I feel like im losing my mind. I have tried suicide attempts before, but always stopped myself. I self harm, because thats the only way I can get my anger and sadness out. I mess everything up, I got cheated on by a lad I loved so much, he cheated on me the day my great nan died, and I was a mess. I just dont know how to cope anymore