Note how specific the passages are about where the work happens. It is at the moment of assent, before the reaction, not after it. A paraphrase that relocates it to "how you react" is describing something else: see did Epictetus say it is not what happens to you?
It is not events that disturb people, it is their judgments concerning them.
Epictetus, Enchiridion 5
If you are pained by any external thing, it is not this that disturbs you, but your own judgment about it. And it is in your power to wipe out this judgment now.
Marcus Aurelius, Meditations VIII.47
Reject your sense of injury, and the injury itself disappears.
Marcus Aurelius, Meditations IV.7
Cast away opinion: thou art saved. Who then hinders thee from casting it away?
Marcus Aurelius, Meditations XII.25
Things have no hold on the soul. They have no access to it, cannot move or direct it. It is moved and directed by itself alone.
Marcus Aurelius, Meditations IV.12
It is not a man who reviles you or strikes you who insults you, but it is your opinion about these things as being insulting.
Epictetus, Enchiridion 20
If a child of a stranger is dead, immediately you say, 'It is a human accident.' But when your own child is dead, then you cry out, 'Alas! I am the wretched one.' But it ought to be remembered that we feel the same when we hear of it of someone else.
Epictetus, Enchiridion 26
Everything has two handles: the one by which it may be borne, the other by which it may not. If your brother acts unjustly, do not lay hold on the action by the handle of his injustice, for by that it cannot be borne; but rather by the opposite — that he is your brother, that he was brought up with you.
Epictetus, Enchiridion 43
Pain is slight if opinion has added nothing to it. In believing it slight, you will make it slight. Everything depends on opinion.
Seneca, Letters to Lucilius Letter 78
Pain is either bearable or brief. As long as you remember this, and don't expect more, you can endure it. The mind preserves its own serenity by withdrawing from the body.
Marcus Aurelius, Meditations VII.33

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