Is there a Buddhist equivalent to Marcus Aurelius' "Meditations"? What I mean by equivalent is in significance and style. Marcus Aurelius was an extremely important man and was forced to deal with specifically difficult problems in life. This adds weight to his words.

Is there any very famous Buddhist diary? One which is mostly just a person talking to themselves about dealing with their own issues, with philosophical (or religious) insight and vigor? (It's important to note that Marcus Aurelius' Meditations cover very intense stoic philosophy. Serious stuff that is often hard to penetrate without understanding stoic logic and concepts.)

So, in short, a diary kept by a monk which covers many important Buddhist concepts.

It must be a diary, as such, things like the Dhammapada aren't what I'm referring to.
It is not a diary, but There are the letters by Nanavira thera, where he tries to understand the dhamma.
https://www.nanavira.org/pre-sotapatti-stp

he is a famous bikkhu who claimed to have reached stream entry, then commited suicide. SO his letters are seprated according to the date of the alledged stream entry.