Seriously. Writing Is Hard.

I've really been struggling today with my writing. I committed to having a good version of my dissertation proposal to my advisor by Friday. I know what I want to write, but I've been dinking around most of the day not being able to dig in and do it. I don't know why this happens. I know I'm a good writer, I know as I write, I think, and what I turn out ends up being better than it was in my head. I know it's a process. Why is it so hard to get started and stay going??????? It's so frustrating. And depressing.