Bring the funny in

After going through all those negative scenarios in your head, set them free. Try to think about what's the real possibility of all those bad things really happening. Then try to understand what makes you think about those things. Where does it all start?
You won't be able to clear everything from the start, but don't give up.
After some soul searching, there is nothing better to get you out of that negative mood than a good laugh. It's been proven scientifically that laughter really helps. Maybe you don't really feel like watching a comedy, but that's when you should do exactly that. Or have some nice reading time. Did you ever read Three Men in a Boat? 
That book brings my spirit up every time. You can go to the theatre, see a funny play, listen to some happy songs- you can find ready-made playlists, go out with friends for a fun activity, maybe read some jokes. Do something you don't usually do, just make it fun.
What's your favourite way to unwind? What do you do for fun?