In The Philosophy of Cognitive Behavioural Therapy," Donald Robertson discusses the seeming Stoic paradox of being strict mechanical determinists (who believe that all events are caused by preceding events, therefore even decisions we make are predetermined, in a sense) as well as firm believers in free will. The apparent paradox is caused mostly by misunderstanding of the terms involved and their application to the philosophy. But regardless, Robertson shows that the "sage desires only that he should do what he can to the best of his ability, no more and no less, and he accepts success or failure with equal serenity because he concerns himself only with the quality of his actions, and not their results." He quotes Seneca:
"It is in no man's power to have whatever he wants; but he has got it in his power not to wish for what he hasn't got, and cheerfully make the most of the things that do come his way."
Or, as Robertson quotes Epictetus,
"Don't ask things to happen as you wish, but wish them to happen as they do happen, and your life will go smoothly."
This is the idea that Nietzche (a professor of classical languages) labeled "amor fati" - "love of one's fate." This is the idea that kept Stockdale going as a P.O.W. in Viet Nam:
"Was I a victim?Not when I became fully engaged, got into the life of unity with comrades, helping others and being encouraged by them. So many times, I would find myself whispering to myself after an exhilirating wall tap message exchange: 'I am right where I belong; I am right where I was meant to be'."