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.
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?