I try not to fight it. When I catch myself feeling miserable, I take a break and go do something concrete/literal like a chore (if you're home). I also have a couple different "types" of writing projects and places online (like here) where I can go and the shift in venue helps. Figuring out something practical on the computer and feeling like I'm "doing" something helps break the intensity of the thoughts reading and writing. Sometimes just making a list or outline of how I'm going to work on a project, instead of actually working on it is good too.

I highly recommend Heather Sellers' books, Page after Page and Chapter after Chapter. They're fun, encouraging reads and go into the psychology of writing and how difficult it is, etc. http://www.heathersellers.com/