“Occupy your mind with good thoughts, or the enemy will fill them with bad ones.” – attributed to St. Thomas More, Chancellor of England. Unfortunately, I can’t find a primary source for this, hence ‘attributed’.
“A gifted American psychologist has said, ‘Worry is a spasm of the emotion; the mind catches hold of something and will not let it go.’ It is useless to argue with the mind in this condition. The stronger the will, the more futile the task. One can only gently insinuate something else into its convulsive grasp. And if this something else is rightly chosen, if it is really attended by the illumination of another field of interest, gradually, and often quite swiftly, the old undue grip relaxes and the process of recuperation and repair begins. The cultivation of a hobby and new forms of interest is therefore a policy of first importance to a public man.” – Sir Winston Churchill, “Painting as a Pastime“