When disturbing thoughts start to hurt you — and you can't stop them from coming — fight back with your reasoning. Beat them. Don't let them get stronger or drag you into their consequences by painting whatever pictures they want in your mind. If you're stuck on the island of Gyara, don't fantasize about the good life back in Rome — all the pleasures someone had there, all the pleasures you'd have if you returned. Instead, focus on this: how should a brave person live on Gyara? Live like that. And if you're in Rome, don't daydream about life in Athens. Think only about how to live well in Rome.
Then, when the appearance (of things) pains you, for it is not in your power to prevent this, contend against it by the aid of reason, conquer it: do not allow it to gain strength nor to lead you to the consequences by raising images such as it pleases and as it pleases. If you be in Gyara, do not imagine the mode of living at Rome, and how many pleasures there were for him who lived there and how many there would be for him who returned to Rome; but fix your mind on this matter, how a man who lives in Gyara ought to live in Gyara like a man of courage. And if you be in Rome, do not imagine what the life in Athens is, but think only of the life in Rome.