Letters 4.6

For this reason, make life as a whole agreeable to yourself by banishing all worry about it. No good thing renders its possessor happy, unless his mind is reconciled to the possibility of loss; nothing, however, is lost with less discomfort than that which, when lost, cannot be missed. Therefore, encourage and toughen your spirit against the mishaps that afflict even the most powerful.

Seneca, Letters to Lucilius 4.6, translated by Richard Mott Gummere