(wikipedia article)
Epictetus:
"First, decide who you would be. Then, do what you must do."
"When I see a man in a state of anxiety, I say, What can this man want? If he did not want something which is not in his power, how could he still be anxious?"
"Freedom is secured not by the fulfilling of one's desires, but by the removal of desire."
"Nothing outside the will can hinder or harm the will; it can only harm itself. If then we accept this, and, when things go amiss, are inclined to blame ourselves, remembering that judgment alone can disturb our peace and constancy, I swear to you by all the gods that we have made progress."
"If you would not fail of what you seek, or incur what you shun, desire nothing that belongs to others; shun nothing that lies beyond your own control; otherwise you must necessarily be disappointed in what you seek, and incur what you shun."
"In a word, neither death, nor exile, nor pain, nor anything of this kind, is the real cause of our doing or not doing any action, but our opinions and the decisions of our will."
"Where is the good? In the will. Where is the evil? In the will. Where is neither of them? In those things which are independent of the will."
"Who then is the invincible? It is he whom none of the things disturb which are independent of the will."
"Man is disturbed not by things, but by the views he takes of them."
"No man is free who is not master of himself."
"Wherever I go it will be well with me, for it was well with me here, not on account of the place, but of my judgments which I shall carry away with me, for no one can deprive me of these; on the contrary, they alone are my property, and cannot be taken away, and to possess them suffices me wherever I am or whatever I do."
"I am formed by nature for my own good: I am not formed for my own evil."
"If, therefore, any be unhappy, let him remember that he is unhappy by reason of himself alone."
"Every person must deal with each thing according to the opinion that he holds about it."
"Permit nothing to cleave to you that is not your own; nothing to grow to you that may give you agony when it is torn away."
"He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has."
"God is best worshiped in the shrine of the heart by the desire to know and obey him."