Meditations 6.19.1
Marcus Aurelius to himself
Μή, εἴ τι αὐτῷ σοὶ δυσκαταπόνητον, τοῦτο ἀνθρώπῳ ἀδύνατον ὑπολαμβάνειν, ἀλλ’ εἴ τι ἀνθρώπῳ δυνατὸν καὶ οἰκεῖον, τοῦτο καὶ σεαυτῷ ἐφικτὸν νόμιζε.
Do not judge something impossible for a human being because it is hard for you to accomplish. If it is possible and fitting for a human being, consider it attainable for yourself too.