Did Marcus Aurelius invent the personal diary? Were there other people before him?

Dubious. Introspection, meticulousness, vanity and such pertinent traits aren't unique to emperors. Conceptually, it's not much of an invention. The hard part historically was producing such a thing, and preserving it through the ages: Scarcity of writing implements, widespread illiteracy, lack of time to spare in the face of survival, people who deem such mundane prose worthy of constant upkeep & storage (even available means). Hence that's the earliest known 'extant' diary. Someone amidst the antecedent greek philosophers, chinese or egyptian societies would have devised a form of personal documentation for intellectuals to utilise. Besides, it's a matter of definition - there's the diary of merer, it seems a bit less of a diary than logbook.