Perhaps less to do with Karma and more about psychology. Can you keep a diary and think about your life history from an emotional point of view? Try to find out what sort of things make you sad, especially events from your past. Try and deal with them, because if you just don't even think about them, that sadness will stay with you and you won't know why. Also, change your surroundings if you can. A beautiful place to live with lots of nature around can really beat depression. If you can't move, at least try and get yourself somewhere beautiful once a week. (The ocean works best for me when I'm blue, but there are mountains, lakes, hills, waterfalls, etc. that do it too)
Take a peek at this site...maybe it can help a little?
http://www.beyondblue.org.au/ copy
Supplement from 02/17/2007 10:43pm:
Oh yes, and don't worry about it for your friends' sake. You're not here for their benefit. It's about you.