I have so been there. Usually, for me, the answer is to actually give myself permission to do something fun, just for a bit. I watch a sitcom. I knit a little. Read one *short* piece of porny fanfic. Sometimes that's enough to break the deadlock of "can't have fun until I work, can't work until I unlock the brain by having a little fun."

However, if you're in the zone where you actually have no time for the above strategy, then I use this one: If you can't write anything good, then write something bad. Open a separate file if you want to, just so you don't get good and bad mixed up, but put words to the page. Make your first paragraph "I hate this paper and I was an idiot to choose this topic because..." Permission -- nay, the requirement -- to write crap often frees up the logjam of self-criticism and panic that's holding back all the good stuff that you know is in there, because it was there when you thought of this idea.

I have truly been there. I completed grad school while working full time. Well -- I was paid full time. The job wasn't actually all that demanding. But still -- I came out on the far side, degree in hand, and you will too.