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 enormously diminished. Now fear, in its most harmful form, arises where there is some danger which we are unwilling to face. At odd moments horrible thoughts dart into our minds; what they are depends upon the person, but almost everybody had some kind of lurking fear. With one man it is cancer, with another financial ruin, with a third the discovery of some disgraceful secret, a fourth is tormented by jealous suspicions, a fifth is haunted at night by the thought that perhaps the tales of hell-fire told him when he was young may be true. Probably all these people employ the wrong technique for dealing with their fear; whenever it comes into their mind, they try to think of something else; they distract their thoughts with amusement or work, or what not. Now every kind of fears grows worse by not being looked at. The effort of turning away one’s thoughts is a tribute to the horribleness of the spectre from which one is averting one’s gaze; the proper course with every kind of fear is to think about it rationally and calmly, but with great concentration, until it has become completely familiar. In the end familiarity will blunt 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. When you find yourself inclined to brood on anything, no matter what, the best plan always is to think about it even more than you naturally would, until at last its morbid fascination is worn off.
Source: The Conquest of Happiness (book), by Bertrand Russell