This is one of those ideas that provides missing pieces to so many puzzles.

Willpower is limited. It is like a muscle. When you have friction and stress, you need willpower to keep going. The more you use willpower, the less you have left to use, until you can replenish it. This is ego-depletion in a nutshell.

What sucks my willpower?

Doing anything that it outside natural routine
Incessantly checking email to not miss anything
Last minute meetings
Fires to put out
Narcissistic people
Ever-expanding to-do list
Impossible deadlines

How to replenish it? We are still learning how to replenish willpower, though here are ways that have worked for me.

Rest, rest, rest — when my day starts getting tough, I start simplifying the evening
Turn rumination into questions: when my willpower depletes, I start ruminating about hypothetical ways problems will arise. I have started to turn those ruminating thoughts into questions via this TED talk. Having a question to answer energizes me to think of a viable solution instead of losing willpower to suffering in the problem
Reflect on Core Values: Having a simple list of the values that are important can give you a temporary boost
Reflect On Good Outcome of Goals: Not fantasizing but imagining what a good outcome looks like can also help with a temporary boos to willpower

What sucks your willpower? How do you replenish it?

For more information on this topic, see paper "Ego Depletion: Is the Active Self a Limited Resource?", 2000 by Baumeister, Bratslavsky, Muraven and Tice