I don't know what's wrong!!?

I sit and stare into space like I'm in a constant daydream. I get really upset, on the brink of tears and sometimes I do cry. It all stems from one problem (which I can't explain) and some days I can feel quite happy and then its like reality has kicked back in and I feel bad because I haven't been thinking about my problems. I just feel really down and when I want to cry I can't. I put on a brave face and people think I'm fine, but inside its tearing me apart. I have talked to a few people about this(so no suggestions about talking coz its got me nowhere). Please help, what should I do, does anyone know whats wrong with me?