Letters 24.2

But I shall conduct you to peace of mind by another route: if you would put off all worry, assume that what you fear may happen will certainly happen in any event; whatever the trouble may be, measure it in your own mind, and estimate the amount of your fear. You will thus understand that what you fear is either insignificant or short-lived.

Seneca, Letters to Lucilius 24.2, translated by Richard Mott Gummere