People are commenting 'fates worse than death' and 'being made into a labrat by the 1%', but really, if you have infinite time to just do stuff and you can't be killed -- And you don't somehow squirrel your way into a position of power then what are you even doing with your time and immortality, oomfie?

The loneliness part is also questionable. I know OP said it's overly done, but I also think it's just wrong. If you're an adult you've had people in your life die before. It sucks. You miss them. But then you move on. And you meet other people. You'll still go ":(" when you think about the person and such... But life goes on.

And that's just life. It doesn't get any worse if you extend it longer -- If anything it gets better. You might have lost your beloved today, but you have another dozen lifetimes to heal your wounds and meet someone else and fall in love again and (...)

So here's some lower-stakes, frustrating inconveniences of being immortal:

• Your favourite fashion? It's not just out of fashion. It's so out of fashion it is now considered 'historical costuming'. You can no longer find any articles like it at all. Because the only people even trying to recreate the techniques are costuming nerds and theater people who always exaggerate stuff
• You got a song stuck in your head. It is either from before recording was invented, or any recordings of it that existed are too old to be reliably listenable. You have a song stuck in your head.
• You used to really enjoy a job you did. That entire career path is now obsolete. As per the first paragraph of my post, if you're immortal you have probably snuck your way into the upper echelons of society at some point during your infinite time... But like. You're bored. You loved being a Court Jester, now there are no Court Jesters.
• Actually tedium just in general. Sooner or later you'll run out of new things to try, because you'll have done everything that even remotely caught your eye already. So what the fuck will you do with your time? You'll eventually just get depressed and not do anything.