I would suggest talking to a counselor to help figure out why you're feeling that way. If your work offers EAP (which may be a legal requirement), the counseling should be free.

There's also stuff you can search for on the net like this... http://freshmint4u.wordpress.com/2007/06/19/9-ways-to-stop-being-negative/

Or there's probably a bunch of self help books you could look into.