Here's a little trick for those days or weeks when it seems like your to-do list has its own to-do list and there's no conceivable way to get it all done:
Work in 90 minute blocks. Then chill for 15 minutes.
It doesn't matter what you do for those 15 minutes, so as long as it's not work related. I did this in college. It worked. Fabulously.
I do the same thing today. 90 minutes of focused work followed by 15 minutes of fly-tying. Or a walk in the garden. Or sketching. You get the idea.
Though it may feel contrary to logic, I get more done this way than when I try to power through, non-stop.
Play with the formula. Find a version that works for you.