I'm easy to feel blue.
According to the degree of depression, there are different solutions:
#1 too lazy to work: Maybe one day, but not today
#2 too sick to feel happy: Routine for unproductivity
#3 too upset to cheer up: Upset? Play this slide
Now, I'll introduce #4, the true blues, too disappointed about myself. (@brianball you read the above three, in case you'll be interested in #4, too )
The above three won't work on #4, because I still get things done (#1), talk to others in the smile (#2), focus on my to-dos (#3), but I'm dead inside.
When it occurs (like now), my soul left with a message "You don't know anything", then daily jobs would reply on self-discipline to schedule.
How to comfort my spirit?
I read math books, math books in my kindle, and only the beginning.
Usually, the first chapter is preface or introduction for readers to keep them reading.
For math books, there is a more significant mission: convince readers that you could discover the beauty and elegance of math world, no matter how old you are and what you are living for.
Here are three of my favorites:
Different from TED talks, self-help books, quotes, and films which activate you too excited by audio or video, these chapters are talking to you quietly, "you could understand the law of nature clearly, just be patient, trust me. "
(If you're interested, here are the first chapter of the three books.)