you can't help it. some people are really very sensitive. Accept it. Try to divert your attention to the beautiful things of your life. Recall the beautiful moments of your life. Try to focus on your studies( if you are a student) or your work(if you are working). Give yourself a break from stress. Find someone who will hear out your feelings or you can just start writing your own diary. When you pour out your feelings onto someone else, you feel really better. Just give it a try.
I was like that through much of my 8th grade year. I didn't have many friends, I was out of shape, and all if the girls I liked didn't like me back, and my grades were horrible. I was always angry or sad, but I never really knew why. But thing just sort of got better, I don't know how or why, but when I went to high school, I got back into shape, made more friends, got a girlfriend, and overall just started anew. My recommendation would be to just give it time, stay at least semi fit, keep you graded good, but also focus on your social life go to church. Just give it time, and you can turn things around.
You have some good answers here but don't forget proper nutrition also. It is amazing what dropping junk food, processed, refined foods, and 'white' foods can do for you. (white as in rice, sugar, flour etc,...) Doc Joel Wallach has recommendations that can help with this. YouTube and Youngevity.com