Effort is crucial in the beginning for generating a strong will. We all have the Buddha nature and thus already have within us the substances through which, when we meet with the proper conditions, we can turn into a fully enlightened being having all beneficial attributes and devoid of all faults. The very root of failure in our lives is to think, “Oh, how useless and powerless I am!” It is important to have a strong force of mind thinking, “I can do it,” this not being mixed with pride or any other afflictive emotion.
Moderate effort over a long period of time is important, no matter what you are trying to do. One brings failure on oneself by working extremely hard at the beginning, attempting to do too much, and then giving it all up after a short time. A constant stream of moderate effort is needed. Similarly, when meditating, you need to be skillful by having frequent, short sessions; it is more important that the session be of good quality than that it be long.
From Kindness, Clarity, and Insight, pages 50–51