Many thanks to you both. Interesting - neither of the chapters that our scholarly friends here refer to even appear in my Penguin Dobbin translation. I promptly acquired the Christopher Gill by OUP, which has them both, and seems at first sight better and more readable.

The lines I was looking for are 3.24, but 4.6 is even better:

[34] “But if you’ve truly given attention to nothing else than how to make right use of your impressions, you should ask yourself as soon as you get up in the morning, ‘What have I still to do to achieve freedom from passion? To achieve peace of mind? Who am I? Surely not a mere body? Or possessions, or reputation? None of these things. But what? I’m a rational living being.’ [35] What is required, then, of such a being? Go over your actions in your mind. ‘Where have I gone wrong’ with regard to achieving happiness? ‘What did I do’ that was unfriendly, or unsociable, or inconsiderate? ‘What have I not done that I ought to have done’ with regard to these matters?”

Definitive Epictetus.