i used to be the same, when i was at school and now sometimes im the same for work! I mean i dont burst out crying, but i get soooo down!
i realised that it is sometimes because there is a certain something i am dreading! like an exam i havent studied, or a guy i dont want to meet, or maby a project i havent completed!
Look into the problem and see what is causing it! If u still cant find the cause, than just think of spending time with your firends, and that in a way school can be fun!

It sometimes worked for me!

Good luck!