This is totally true. I find when I am most depressed and irritable about work is when I have become too engaged in it.

Pulling back and seeing it as just work, just time you are selling to someone else, clears my head and paradoxically makes me a better co-worker both in attitude and performance. Then I slowly get drawn back in and depressed again.

Its a cycle, but at least I can see it happening now rather than just getting burnt out.