I don't think any one source made it click for me, but I think some combination of watching The Good Place, Sandman, and a lot of Black Mirror got me really stretching my imagination of what it would feel like to be truly immortal. I had a moment that felt like my horizons had been expanded very slightly when I felt this severe dread for maybe half a second. A feeling of being inescapably trapped.
There's also this PC game called The Coin Game that's just a solo-dev making lots of arcade games. They exist on an island where you have a home and some hobbies and a few arcades and I think even a mall. But the entire island is devoid of humanity. There's just a bunch of robots. I don't know if the game has a backstory, but the one my brain filled in is that this is a sort of playground for you to live in forever... and it's got a San Junipero feel, but far more bleak. Gave me the chills. I'm happy to be mortal.