Seneca, Moral Epistles 18.5-6
“Still, it pleases me so much to test the strength of your conviction that I will select for you a lesson from the great men: designate some days for yourself when you will be happy with the cheapest food, with hard and poorly-made clothing, and say to yourself: ‘Is this something I was afraid of?”
The mind should prepare itself for difficulties amid safety and harden itself against harm during the enjoyment of good fortune. The soldier practices in the middle of peace, builds fortifications without any enemy around, and wears himself out with meaningless work in order to to meet necessary challenges.
If you don’t want someone to hesitate in the act itself, you need to train before it happens. This is the practice of those who have approached total privation by imitating poverty every month so that they might never grow pale at something they had often studied.”
Ceterum adeo mihi placet temptare animi tui firmitatem, ut ex praecepto magnorum virorum tibi quoque praecipiam: interponas aliquot dies, quibus contentus minimo ac vilissimo cibo, dura atque horrida veste dicas tibi: “Hoc est quod timebatur?” In ipsa securitate animus ad difficilia se praeparet et contra iniurias fortunae inter beneficia firmetur. Miles in media pace decurrit, sine ullo hoste vallum iacit et supervacuo labore lassatur, ut sufficere necessario possit. Quem in ipsa re trepidare nolueris, ante rem exerceas. Hoc secuti sunt, qui omnibus mensibus paupertatem imitati prope ad inopiam accesserunt, ne umquam expavescerent quod saepe didicissent.