You've got a canister. In it is a devastating combination of your Will and Attention. These are limited resources. Directing all of your mental, not to mention physical ("not to mention" an extremely peculiar phrase), energy toward one task is incredibly taxing. I'm a pretty extreme guy, but it doesn't matter how well intentioned you are, sometimes you use up everything in your little can secret stuff. I'm swimming in an ocean of my own blood, sweat and vomit. These are the elements that take over when you're running on empty. It's as bad as it sounds.
Uber Sapiens don't distance themselves from the utter despair of painful practice. While punching the heavy bag for multiple sets of 6 minutes and doing other unsightly drills, I found myself wishing away the time. I was daydreaming of pokemon and pretending my left arm wasn't on fire. It helped a little. What if I seized the moment, in spite of the tiredness, what if I focused on perfect technique, focused on what I was doing instead of hoping and praying to gods that I don't believe in that it would all be mercifully over as soon as possible? Well, then I'd be amazing. So many wasted moments, so much to learn grasshopper.