As far as I’m concerned, R2’s memory was never wiped. This means he has experienced EVERYTHING from Ep 1 to Ep 9 and remembers it ALL. He watched Anakin grow into a Jedi, watched him raise Ahsoka and serve in the Republic-Separatist conflict, fall into darkness and Father a child with Padme who then dies.
He then has to serve on the side that serves against Anakin, definitely hearing of the tragedies and horrors Anakin commits as Darth Vader. Witnesses C3-POs memory being wiped multiple times, essentially losing his closest friend and having to build their relationship from the ground up multiple times. He serves with Luke and Leia, watching them grow into fine adults, witnessing Master Yoda die, Obi-Wan too, then Luke and Leia aswell.
A lot happens to R2 and i can’t stop thinking about it