From Rohan Healey's Greeks to Geeks: Practical Stoicism for the 21st Century:
"Stoic Warriors: Making the Best of a Bad Situation
Epictetus: 'If you wish your children, and your wife, and your friends to live forever, you are stupid; for you wish to be in control of things which you cannot, you wish for things that belong to others to be your own. So likewise, if you wish your servant to be without fault, you are a fool; for you wish vice not to be vice, but something else. But, if you wish to have your desires undisappointed, this is in your own control. Exercise, therefore, what is in your control. He is the master of every other person who is able to confer or remove whatever that person wishes either to have or to avoid. Whoever, then, would be free, let him wish nothing, let him decline nothing, which depends on others, else he must necessarily be a slave.'
You may be thinking, 'That's all well and good, going about telling people not to care about what they don't control, but is that even possible? It's so hard! It couldn't possibly actually work, could it? It can and does work . . ."
Healey goes on to give examples from the life of Vice-Admiral James B. Stockdale. He cites some quotes from Stockdale, including:
"You must never confuse faith that you will prevail in the end - which you can never afford to lose - with the discipline to confront the most brutal facts of your current reality, whatever they might be."