I'm not a therapist, but I can share a couple of things that have worked for me.

-Express it. I know that's a cliché answer, but it really does help. When I get too anxious, it can become a negative feedback loop, but writing about the problem forces it out of my non-linear thoughts and puts it in a linear, more manageable context. It also gives me the basic satisfaction of having dealt with it in a productive, or at least not-unproductive, way.
-On that note, write messages that you aren't going to send to their recipients. Bare your soul if you feel like it. You can turn that dialogue into self-dialogue and start addressing the problem from a different angle. It's like flipping a coin to make a decision: the important part isn't actually the result of the coin flip, but the act of flipping forcing your brain to process things differently.

Usually when I'm in a bad mood I just shut down, ends up wasting my whole day.