(Warning: lots of text, but crucial to read) Please help me... what should I do...?

(I'm fifteen... if that makes a difference) Lately, I've been having very dark thoughts. I can hardly go a day without thinking about killing myself. The ways I could do it, where I would do it, when I could pull it off, what I would leave behind... these frequently cross my mind. My emotions have me so confused... I want to die, my life as a whole holds little worth to me, and I feel I have let down everyone around me. The only thing that keeps me from feeling like a total failure is my talent in the arts, but the arts are a bad field for living and won’t get me through life, so what’s the use anyway? -___- I feel like, if I were to die, my absence would not affect many (My family, yes, I believe they would cry a lot T_T you, however, make no difference. don’t say you will miss me. You don’t even know me…) but, despite my hatred of what I am, I can't, and will not kill myself. It would hurt my family too much (my dad lost his dad about two years ago. I’d imagine it still hurts him some…), I'm a wimp and can barely handle a paper cut (so how could I handle a deep wound >_<), and my religion declares the act a moral sin (probably going to hell anyway, but whatever…). Still, these thoughts are relentless. Maybe it would be a good idea to talk to someone or see a professional, but I cannot. I'm pretty sure I have something like social anxiety (or at least extreme shyness to the point where I can't meet new people. My grades may even be suffering from it) and I am terrified of making phone calls (so no suicide hotline for me either) and talking to strangers and making eye contact. I'm too afraid to go to my parents, don't want them to know anyway, and don't want to bring up my sadness with my friends. My friends are also a problem... I used to have a good amount, but my older friends have grown apart from me and I feel more and more alienated with my current friends (especially when their other friends are around… I get stiff and quiet…) to the point where I feel like within a year they will not talk to me anymore (which is actually probably not true. They talk to me like all their other friends… I’m just very subconscious I guess…). School also has me pretty down. We have so many group projects and activities, and, with none of my close friends in my cles, I find it very hard to cope. I always leave myself as the last possible person without a group, praying someone will pick me or for the teacher to ign me to a group rather than pick one myself (I don’t fit in with any cliques in my cles…). Sometimes I do group activities completely by myself, sitting close to large groups praying the teacher doesn’t ask me what group I am in. These instances make me want to cry sometimes and I get hot, only relieved when the bell dismisses me from the cl. I want so badly for the teachers or someone to know I might have SAD, and there's a reason why my cl participation is worse than average, but I’m so nervous >_< For some reason I just don’t want my parents to know… (I’ve thought about telling my mom on many occasions, but I can’t build up the nerve…)These days I am only happy when I am with my friends or am being distracted by humor or a light conversation with family, but the minute I am secluded from cheer of others the happiness fades and I feel stupid for daring to smile. My life is good. People would kill for my life. I just can’t stand the sadness though. My mind deeply wants to overdose pills or something, but I’m too scared to do anything >_< Too scared to hurt myself, too scared to help myself… what do I do…? Please help me… before my logic dissipates and I do something I could regret...