Dr. James Gordon Gilkey preached a sermon in 1944 called "Gaining Emotional Poise," which was reprinted in 'Reader's Digest' and became a classic almost overnight. He had found, through many years of counseling, that one of the main causes of breakdown, worry, and all sorts of other personal problems, was this bad mental habit of feeling that you should be doing many things now. Looking at the hourglass on his desk, he had an inspiration. Just as only one grain of sand could pass through the hourglass, so could we only do one thing, at a time. It is not the job, but the way we insist on thinking of the job that causes the trouble.
Most of us feel hurried and harried, said Dr. Gilkey, because we form a false mental picture of our duties, obligations, and responsibilities. There seem to be a dozen different things pressing in on us at any given moment, a dozen different things to do; a dozen different problems to solve; a dozen different strains to endure. No matter how hurried or harried our existence may be, said Dr. Gilkey, this mental picture is entirely false. Even on the busiest day the crowded hours come to us one moment at a time; no matter how many problems, tasks, or strains we face, they always come to us in single file, which is the only way they can come. To get a true mental picture, he suggested visualizing an hourglass, with the many grains of sand dropping 'one by one.' This mental picture will bring emotional poise, just as the false mental picture will bring emotional unrest.
Try to Do Only One Thing at a Time (Psycho-Cybernetics by Maxwell Maltz, MD, FICS)
Another similar mental device that I have found very helpful to my patients is telling them: "Your Success Mechanism can help you do any job, perform any task, solve any problem. Think of yourself as 'feeding' jobs and problems to your Success Mechanism as a scientist 'feeds' a problem to a computer. The 'hopper' to your Success Mechanism can handle only one job at a time. Just as a computer cannot give the right answer if three different problems are mixed up and fed in at the same time, neither can your own Success Mechanism. Ease off on the pressure. Stop trying to cram into the machinery more than one job at a time."