Enchiridion · §5

ταράσσει τοὺς ἀνθρώπους οὐ τὰ πράγματα, ἀλλὰ τὰ περὶ τῶν πραγμάτων δόγματα· οἷον ὁ θάνατος οὐδὲν δεινόν, ἐπεὶ καὶ Σωκράτει ἂν ἐδόκει δεινόν, ἀλλὰ τὸ δόγμα τὸ περὶ τοῦ θανάτου, ὅτι δεινόν, ἐκεῖνο τὸ δεινόν ἐστιν.ὅταν οὖν ἐμποδιζώμεθα ἢ ταρασσώμεθα ἢ λυπώμεθα, μηδέποτε ἄλλον αἰτιώμεθα, ἀλλ᾽ ἑαυτούς, τοῦτ᾽ ἔστι τὰ ἑαυτῶν δόγματα. ἀπαιδεύτου ἔργον τὸ ἐγκαλεῖν ἄλλοις ἐφ᾽ οἷς αὐτὸς πράσσει κακῶς· ἠργμένου παιδεύεσθαι τὸ ἐγκαλεῖν ἑαυτῷ· πεπαιδευμένου τὸ μήτε ἄλλῳ μήτε ἑαυτῷ.

tarassei tous anthrōpous ou ta pragmata, alla ta peri tōn pragmatōn dogmata; hoion ho thanatos ouden deinon, epei kai Sōkratei an edokei deinon, alla to dogma to peri tou thanatou, hoti deinon, ekeino to deinon estin. hotan oun empodizōmetha ē tarassōmetha ē lupōmetha, mēdepote allon aitiōmetha, all' heautous, tout' esti ta heautōn dogmata.

It is not things that disturb people, but their judgments about things. For example, death is nothing terrible — if it were, Socrates would have thought it terrible — but the judgment that death is terrible: that is what is terrible. So whenever we are obstructed, disturbed, or grieved, let us never blame another person, but ourselves — that is, our own judgments. It is the mark of an uninstructed person to blame others for their own troubles; of a person beginning instruction, to blame themselves; of a fully instructed person, to blame neither others nor themselves.

— Enchiridion 5

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation