I've gone through phases of real pessimism, so I know what it's like! Poor you *hug* I hope some of my advice helps and you have a bubbly glass once more!
1. Treat yourself. Do things that make you smile. Watch a funny film, chat to a friend, listen to happy songs, reach an acheivement or just bake and eat the most amazing triple chocolate, greatly fattening but amazingly yummy brownies!
2. Avoid sad stuff. This may sound obvious, but I find with stuff I normally enjoy - say music - I pick up the mood without realising. So avoid sad stuff, and don't try and force yourself to act "normally" cos it may just highlight how different you're feeling.
3. Notice one good thing every day. Write it down, and as you add to your list, read through it. Really make an effort. Quote nice things people say; watch people on the bus to see if they do something sweet or funny; remember that the flower you stopped to sniff in that hedge smelt gorgeous.
4. If there's specific stuff you're stressed about, here's my method. I put on my trainers, grab a pen and paper and go for a run. (I get twitchy and full of adrenaline when I'm anxious!!) I get to the nearest body of water (a stream about 20 minutes from my house) and write about what I'm feeling. Or if it's something a friend has done, sometimes I write them a letter. I hold it in my hands and imagine putting the problem onto the sheet. (I'm a Christian, so here I normally pray too
) Then I throw the paper - and my problem - into the water and watch them float AWAY from me. Then turn my back and walk away without looking back.
Hope you're feeling better soon.