I'm never happy with myself...?

It's like... Life is so complicated, I get good grades according to my parents but they never feel good enough. I'm short, fat and ugly. I hate when things go even a little bit wrong and when they do I cut myself, which I did two years ago, stopped, and just started again when school started. I got put into the wrong math cl by accident and was stuck in it for two weeks and I felt stupid just for being put in there, which made me cut, obviously. We had to try on scrubs so the teacher could order them and I was so embarred I nearly cried, even though I'm not that big... I ended up being a size medium... I'm just not normally happy, but then when something good happens, I'm completely hyper, happy and overjoyed. Even if I just meet someone who's nice I'm really excited and happy. I don't know, it's like sometimes I hate myself, and other times I love life... I don't know, I guess everyone goes through this in their lifetime, but it's been too long... Help?