I have felt this before. In many ways I know to expect it. But it doesn't make it any easier. The fact of the matter is designing makes me depressed. Why? Why! Damn, I don't like it. But can I at least understand it? Lets see, I am alone in a small room focused on one task for hours and hours. And that task is both mentally difficult and tedious. And the whole time my brain is trying to convince me to flee. But when I take a break I have no idea what I want to do and nothing sounds good. So I am at turns enslaved and adrift.
Wow, That was helpful. Knowing there is a perfectly good reason for feeling bad allows me to feel a little better.