A passage from Epictetus
Epictetus, Discourses III.25
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Has it not been made plain that we should not value the things that are not in our power, but only those that are? And the only things in our power are our judgments, impulses, desires, and aversions.
Related passages
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Men seek retreats for themselves, houses in the country, sea-shores, and mountains... But this is altogether a mark of the most common sort of men, for it is in thy power whenever thou shalt choose to retire into thyself.
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Take away thy opinion, and then there is taken away the complaint, 'I have been harmed.' Take away the complaint, and the harm is taken away.
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At dawn, when you have trouble getting out of bed, tell yourself: 'I have to go to work — as a human being.'