What is wrong with me? Am I depressed?

Hello, I am a 15 year old girl, I live in London and I think I may have depression. I look like an ogre and I genuinely am disgusted with myself for allowing myself get so disgustingly large and hideous. Moreover, I live with my grandmother, who doesn't give a **** about me. She rarely makes interaction with me and knows nothing about me, and I have lived with her since I was 4. I use to live with my parents and my 4 siblings. I no longer live with any of them because my parents were drug addicts and couldn't give a **** about us so the social workers came and took us away and that is all I really remember. I see my siblings once a month and my parents, never. They are still alive, they just don't give a **** about me. Now that I'm 15, I have a greater understanding of my childhood, my parents were drug addicts, my father cheated on my mother and abused her and I guess it was not safe to live with them any longer. My grandmother has cut off all contact with my parents, they both still live in London but just leading their own shameful lives I guess. My mother doesn't even have a proper house, she just goes around and stays with her friends or whatever, I don't really know. She gave birth to my youngest sister 7 years ago when she was in rehab, last year she left her here with my grandmother and I and said she was "going to the shops", she never came back, she just abandoned my sister here, and that is heartbreaking. My family is so difficult so apologies if none of it makes sense. But it just breaks my heart knowing that history has repeated myself, I am so ashamed of the parents I have. Imagine what my sister says to her friends when they ask where her parents are, "I don't know who my dad is and my mum just left me and I don't know why". I know I am fortunate to live with my grandmother but I honestly feel as if nobody gives a damn about me. I feel so worthless and I feel as if my existence is just ever so pointless. Having not seen my parents for years now, I have realised that I am the ugliest human being on earth. So unwanted and unloved. I bottle everything up. I just get upset all the time over nothing, I feel so empty, but there was nothing there in the first place. I have attempted suicide many times, and I have thought about it numerous times. I really do not know what to do anymore, I just want to kill myself and never ever wake up. I have friends but they will never ever understand me or my past. I dont even understand myself. I self harm regularly, and it is pointless I know. I do not know what I am doing with my life. I just hate myself so much. I have been feeling like this for the past 4 years now. Its getting to the point where I have no motivation for anything at all, where I find enjoyment in sleeping, but disappointment in waking up, I am not interested in anything, I hate myself so much, I am so ugly and disgusting, nobody cares about me, I want to die. I use to have a therapist in school because I skipped school behind my carer's back (my gran) for about 3 weeks, during those 3 weeks I walked around town aimlessly hoping to get hit by a car. Therapy was useless. Nobody knows about my self harm. I am just so fed up with life. What do I do? I really do not know anymore. I know I sound like a stupid teenager with problems but please please please just help me. I do not want to go see a doctor or GP, I will look stupid and I do not want to tell anyone about how I feel. I just want to die, rid the world of myself because nobody would care if I died tonight. Please tell me what to do, I am begging you.