Fortify your mind. Seneca, Epistles 2.13.1-3

Test your strength against Fortune. Only then will you find the strength latent in your mind.

Multum tibi esse animi scio; nam etiam antequam instrueres te praeceptis salutaribus et dura vincentibus, satis adversus fortunam placebas tibi, et multo magis postquam cum illa manum conseruisti viresque expertus es tuas, quae numquam certam dare fiduciam sui possunt nisi cum multae difficultates hinc et illinc apparuerunt, aliquando vero et propius accesserunt. Sic verus ille animus et in alienum non venturus arbitrium probatur; haec eius obrussa est. Non potest athleta magnos spiritus ad certamen afferre qui numquam suggillatus est: ille qui sanguinem suum vidit, cuius dentes crepuere sub pugno, ille qui subplantatus adversarium toto tulit corpore nec proiecit animum proiectus, qui quotiens cecidit contumacior resurrexit, cum magna spe descendit ad pugnam. Ergo, ut similitudinem istam prosequar, saepe iam fortuna supra te fuit, nec tamen tradidisti te, sed subsiluisti et acrior constitisti. Multum enim adicit sibi virtus lacessita; tamen, si tibi videtur, accipe a me auxilia quibus munire te possis.

I know you have a great spirit, for even before you made a habit of equipping yourself with healthy precepts, the kind that conquer hard things, already you were content with yourself, though Fortune had turned against you. Your position is even better since you joined battle with her directly, feeling your own strength against her hand-to-hand. Strength is something we can never really trust until it has arisen to meet a host of trials, proving its ability to follow close as difficulties appear now here, now there, now right up in our faces. Then the true spirit reveals its mettle, showing that it will not abandon us to decisions not our own. Present danger is the test by which we prove it. An athlete who has never been injured cannot bring great courage to any contest. The one who has seen his own blood, who has heard his own teeth rattle at the blow of another's fist, whose body has borne the full weight of an adversary throwing him but not his heart, who has ever risen more obstinate than he fell: this one comes down to fight with great hope. So, to finish my metaphor, Fortune has often had the upper hand of you, but you have not yet surrendered; instead, you have leapt up and returned fiercer to the fray. Virtue adds much to herself when she has been wounded; nevertheless, if it seem good to you, accept from me some words of counsel, battle-tested troops with which to fortify your mind.