I also recall a science fiction take on the theme. It was a short story by Stanisław Lem. There's this mad scientist named Decantor, who killed / immortalized his wife by copying her mind onto an indestructible crystal.

The consciousness persists and the mind keeps on thinking, and it can't die - it's virtually immortal. That's the extra twist here. So, it's capable of surviving for millions of years, likely outliving all life on the planet etc., but at the same time deprived of all senses and cut off from the external world. Even an escape into insanity is impossible (since, I assume, there's no biochemistry that could get out of balance).

The horrified narrator considers it as the worst torment imaginable, whereas Decantor can't see it, as immortality - he points out - has always been humanity's most profound longing.