A passage from Epictetus
Epictetus, Discourses I.18
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For to fall into error unwillingly is a man's misfortune. He who suffers pain, then, is not the sufferer of the injury, but the doer of it.
Related passages
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Begin the morning by saying to thyself, I shall meet with the busy-body, the ungrateful, arrogant, deceitful, envious, unsocial.
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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.'