The power to concentrate is a gift. I believe that being able to sustain it is largely the result of self-confidence. If you doubt the power of your intellect or doubt the quality or value of what you are doing you are much less willing to “stay with it.” You’ll find that what you’re doing is a chore, a painful, grinding obligation. In these circumstances concentrating is a task, an unpleasant struggle that actually, physically hurts. There’s always a part of you that is begging for relief, ready to jump ship at the slightest prompt, cut yourself loose from the humiliating confrontation of your pitifully weak brain with the intractable material you’re trying to manipulate and create meaning out of.

John Adams, “Concentration”