Is there a Buddhist equivalent to Marcus Aurelius' "Meditations"?
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.