F = Five More Rule
There are two kinds of people -- those who have learned how to work through frustration, and those who wish they had. From now on, if you're in the middle of a task and tempted to give up -- just do FIVE MORE.
Read FIVE MORE pages. Finish FIVE MORE math problems. Work FIVE MORE minutes. Just as athletes build physical stamina by pushing past the point of exhaustion, you can build mental stamina by pushing past the point of frustration.
Just as runners get their second wind by not giving up when their body initially protests, you can get your "second mind" by not giving up when your willpower initially protests. Continuing to concentrate when your brain is tired is the key to stretching your attention span and building mental endurance.