I feel the same way many days.
It's a good thing I do not have GOD-like super powers, because it would turn out to be a very bad day for planet earth.
I can't give any advice on being more positive, not that you'd want to hear it, I know when I'm feeling negative, the last thing I want to hear is people talking about positive crap.
So, I suggest listening to some dark music of your choice and sit back and imagine asteroids hitting the earth, or maybe look up some hydrogen bomb test footage on youtube
Another fun thing for a dark mood is a good game of Postal 2, try the cat silencer for the automatic weapons, or just go on a rampage with the shovel.
If you are feeling really twisted, manhunt takes the cake for sick games.
Depressive black metal is a great genre if one is feeling depressed, at least in my opinion...and yeah hearing people talk about a bunch of positive crap is not something I enjoy when i feel like that.