Worry is a form of fear, and all forms of fear produce fatigue. A man who has learnt not to feel fear will find the fatigue of daily life greatly diminished.

The proper course with every kind of fear is to think about it rationally and calmly, but with great concentration, until it becomes completely familiar. In the end familiarity will bunt its terrors; the whole subject will become boring and our thoughts will turn away from it, not as formerly, by an effort of will, but through mere lack of interest in the topic. 

Both quotes by Bertrand Russell 

Book: The Philosophy of Cognitive Behavioral Therapy: Stoic Philosophy as Rational and Cognitive Psychotherapy by Donald Robertson