Asklemmy

I think you're undervaluing loneliness. Loneliness isn't just missing some one. Loneliness means there's no point in connecting with people because they will just die. Loneliness means that no one knows the depth of your condition because it isn't available to them. It means that as they change and face new obstacles, you'll be oblivious to all of that. You'll not only see them die, you'll see the vitality deep out of their pores as they age. All the while you'll never know what that means personally or feel that slow slipping.

Also, super weird that your example is a breakup and people dying is something not worth registering.

outlasting humanity and all its trappings

Either "Boredom: After some time you have seen basically everything." or "Can't keep up: The world changes so fast, and I'm, stuck in a mindset I acquired in 1543".

And: Bureaucratic nightmare. "We have you on file as being born in 1924, but you don't really look like a centennial. Can I see your passport instead of that of your great-grandfather, please?"

Repeated surgical corrections for your ever growing earlobes

If everyone gets to be immortal, imagine never being able to get rid of dictators. Putin's 600th won election.

People in the future wouldn't be allowed to have children, Earth will be filled to the brim with very old people and very few new ideas.

I suppose it depends on the rules of this specfic immortality. As someone who lives with chronic pain that literally never feels physically comfortable in any position, immortality sounds like a cruel joke. Not that I'm suicidal or eager to die, but the fact that it would progressively get worse and worse without any sort of end is.... horrorific.

If it's the realistic kind where you just don't age, the statistical certainty that you'll eventually die in an accident, or to war or murder. Your odds of getting to the heat death of the universe without making backups is pretty slim.

If it's the kind where you're indestructible, you're highly likely to encounter someone who tries to bury you alive in a subduction zone eventually, because humans are like that, and then you get to spend eternity slowly moving into the scorching mantle.

How much more annoying the (much) younger generations would be.

Getting trapped under something for a few thousand years.

People, corporations, and other entities would over time gather more data about you. There's always some kind of information footprint that you leave behind. And you'd stand out from other humans by the way you talk (i.e. using slang from 200 years ago, and speaking about historic stuff with details that the general public is not aware of) and other traits, which makes you traceable.

You'd procrastinate things for 100s of years, until at one point you're simply no longer able to do it. Wanted to domesticate a saber-tooth cat some day? Too bad, they're extinct now. Wanted to visit the baths in ancient Rome? Well, it is not the same Rome anymore, and all the baths' floors are cold.

The rest of humanity will eventually evolve into something you don't recognize and can never be part of.