My reasoning on living forever has long been that there are exactly two possibilities:

At some point your mind stops changing at all, or are stuck in some form of loop forever.
Your mind keeps changing forever, and so must eventually completely diverge from the person you started out as.

Both are existentally terrifying, so I'm glad it's only a thought experient. Apparenlty this is called Apeirophobia and afflicts the reliigous more viscerally.

I'd happily take a thousand years to think it over some more. ;)