Like any seemingly insurmountable task
Divide it up into small chunks and deal with them one at a time. You will get there eventually. I have to pet the black dog* on my shoulder sometimes. We all do, it's how you choose to deal with it that defines the outcome.

Get professional help for anything serious. The Internets have been known to give bad advice from time to time :)

Simplify your life. Most importantly though, keep your chin up. Let all the minor, irritating troubles wash over you; they don't matter, things are not as bad as they seem. Stop giving a shit about unimportant things. Start giving a shit about everything else. If you have no control over it, don't worry about it. If you can do something about it, then do it. Find something healthy that gives you real joy.

Bottom line: This is all sweeping generalisations and your situation is bound to be complex and unique, but the sooner you realise the only person who can get you all the way out of the tunnel and back into daylight is yourself, the better. Don't depend on pills or other people to do it for you, it only treats the symptoms.

*Incidentally, I do have a little black dog for real. She's awesome
( chart cat has an itch, Thu 11 Jun 2009, 4:12, Reply)