Why do I feel this way?

I always feel like life has no purpose and I am always thinking about why life even exists. Every morning I wake up feeling terrible about the way I look and everything about myself, I feel like I don't want to go out in public because of it. Everything is depressing to me and I never feel happy. I see people around me and they are so happy and I wonder why I can't feel how they do. Every time I try, I end up thinking too much about life. I always think about the past, and how great it was but I end up crying because I get sad when I think about it. I have tried listening to music to make me feel better, and I really love music, but I can't listen to it anymore because it makes me feel worse and think about the past no matter what kind of music it is. Does anybody know why I feel like this and how I can help it? I hate feeling this way and crying all the time.