Very happy to find this here. I'm a practicing Stoic, but largely in "stealth-mode", meaning I don't talk to people, other than my wife, about it. Stoicism is a wonderful philosophy for overcoming adversity and, beyond that, finding a deep and persistent joy in life.

It has some interesting similarities to Buddhism but is, in my opinion, much more suitable for engineers. There is a strong emphasis put on logic and understanding the world around you through science. While ancient stoics did base some parts of the philosophy on religion, it is no where near an essential part of the philosophy, whereas buddhism is slightly more difficult (though far from impossible) to decouple from mysticism.

Epictetus is a great source, as are the writings of Marcus Aurelius and Seneca (the younger).

I keep a small list of Marcus Aurelius quotes pinned up by my monitor.

A few that work for me:

"Marcus Aurelius: Be a boxer, not a gladiator, in the way you act on your principles. The gladiator takes up his sword only to put it down again, but the boxer is never without his fist and has only to clench it."

"Remember how long you have procrastinated, and how consistently you have failed to put to good use your suspended sentence from the gods. It is about time you realized the nature of the universe. Your days are numbered. Use them to throw open the windows of your soul to the sun. If you do not, the sun will soon set, and you with it."

"Every hour be firmly resolved, as becomes a Roman and a man, to accomplish the work at hand with fitting and unaffected dignity, goodwill, freedom, and justice. Banish from your thoughts all other considerations. This is possible if you perform each act as if it were your last, rejecting every frivolous distraction, every denial of the rule of reason, every pretentious gesture, vain show, and whining complaint against the decrees of fate."

"Stop jumping off the track. You don’t have time to reread your diaries, or the lives of the ancient Greeks and Romans, or the passages from their writings that you’ve collected for your old age. Throw off vain hope and sprint to the finish. If you care about yourself at all, come to your own aid while there’s still time."

"Be like a rocky promontory against which the restless surf continually pounds; it stands fast while the churning sea is lulled to sleep at its feet. I hear you say, ‘How unlucky that this should happen to me!’ Not at all! Say instead, “How lucky that I am not broken by what has happened and am not afraid of what is about to happen. The same blow might have struck anyone, but not many would have absorbed it without capitulation or complaint. "

"How at this moment am I using my mind? This is a question worth asking all the time. So is this: ‘How do my words and deeds measure up to the ruling reason within me? And who owns this mind of mine anyway? An infant? A boy? A woman? A tyrant? A dumb animal? A wild beast?’"

"How shameful and absurd it is for the spirit to surrender when the body is able to fight on!"

"Everything – horse, vine, anything – exists for a purpose. Is it any wonder? Even Helios the sun-god will say, “I have a job to do,” and the rest of the gods will say the same. So what will you say? “I’m here to have a good time?” The very thought is beneath contempt."

"Attend to the matter at hand, whether it be an object, an action, a moral principle, or the meaning of what is being said. You get what you deserve because you would rather become good tomorrow than do good today."

"In the make-up of a rational being, I can see no virtue incompatible with justice, but I do find a virtue at odds with pleasure: self-control."

And a bit of a longer one:

“In the morning, when you can’t get out of bed, tell yourself: “I’m getting up to do the work only a man can do. How can I possibly hesitate or complain when I’m about to accomplish the task for which I was born? Was I made for lying warm in bed under a pile of blankets? ‘But I enjoy it here’. Was it for enjoyment you were born? Are you designed to act or to be acted upon? Look at the plants, sparrows, ants, spiders and bees, all busy at their work, the work of welding the world. Why should you hesitate to do your part, the part of a man, by obeying the law of your own nature? ‘Yes, but nature allows for rest, too.’ True, but rest – like eating and drinking – has natural limits. Do you disregard those limits as well? I suppose you do, although when it comes to working you are quick to look for limits and do as little as possible. You must dislike yourself. Otherwise, you’d like your nature and the limits it imposes. At the same time, you’d recognize that enjoyment is meant to be found in work too and that those who enjoy their work become totally absorbed in it, often forgetting to eat and drink and seek other forms of enjoyment. Do you think less of your life’s work than the sculptor does his sculpting, the dancer his dancing, the miser h is money, or the star his stardom? They gladly forgo food and sleep to pursue their ends. To you, does the work of building a better society seem less important, less deserving of your devotion?”

That last one is literally the one thing that gets me out of bad some days: "Was I made for lying warm in bed under a pile of blankets?"

On many days I think yes, and I have no problems admitting this (especially during winter).

A question from someone largely ignorant of Stoicism, generated by the last clause in your quote from The Enchiridion:

How does a Stoic change his/her lot in life, given that Stoicism assigns choice of that lot to an externality? Or does that question lack meaning within Stoicism?

Your lot is that you were born a human being, on planet Earth, in this time. You cannot change that. You are, however, fully in control of your thoughts and actions, and can live in the present and make your time as aware, useful and meaningful as possible. If you do that, you will be better in the future and your future self will be grateful to your present self.

I don't believe that 'you' (the reader) are the intended 'author' of the play in that quote. It's not that you get to write your own role, it's that it's your job to play the role you were given, and someone else's job to give you that role.