Everything in existence falls into one of two categories: what is up to you, and what is not.
Marcus Aurelius returns to this separation across every book of the Meditations — not because he had not mastered it, but because the practice of returning to it was itself the discipline. He was governing a vast empire, managing wars, plague, betrayal, and the deaths of children. The temptation to locate wellbeing in outcomes was constant, structurally built into his role, and available every day.
The Meditations are the record of a man refusing that temptation, repeatedly, under conditions designed to make the refusal as difficult as possible.