Losing all of the skills you gain. No matter how good you get at something, after a few centuries you'll have lost your edge. You can also only practice so many things concurrently without giving something up. At some point, years down the line, you might try to ride a bike again and completely fail to do it, or try to sing and fail to hit all the notes that came easily before, or do gymnastics but the muscles you need are underused. It doesn't matter that you spent years mastering every skill, your abilities will degrade over time. You'll never really be able to feel sure about your own abilities except for whatever you've done most recently.

All the comments assume everybody else isn't also immortal. I forget the title and author but there's an old sci fi story (or novel?) about a future where everybody lives for centuries, and they've found that the brain only retains a certain amount of experience. They have long careers, get tired of doing whatever, re-educate and do something else, or even have multiple families they eventually forget about. A couple of the characters are surprised to find out they used to be married like a century earlier. To me that seems vaguely like reincarnation, and I kind of don't hate the idea. I really don't see any downside to that scenario, or even just going on forever.

People are focused on having regrets and negatives that last forever. But buck up li'l camper, you can learn to move on from stuff. And I say this as a dad whose daughter had cancer at age 10 (she survived). It was hell and I wouldn't want to live through that whole period again, but I don't consider it a reason not to want to live forever. The trick is to learn how to cope with these things and not let them outweigh the good experiences you have.

Cancer. So much goddamn cancer. It doesn't matter what kind of immortality you have, you WILL get cancer. Repeatedly. Over and over. Forever.

Cross the wrong people and you end up not dead, but irrecoverable. Cement shoes, buried alive kind of stuff. Cross a different set of wrong people and you become a labrat. To avoid either scenario, you'll be in a constant state of "undocumented" or false-documented which will keep you in a pretty consistent state of poverty.

Wage slaving never stops

Having to listen to that Queen song, forever.

The disappointment of experience winning lifetime supply of something but that would eventually turn into a lie

Being asked your birthdate in order to view a game on Steam, and the year dropdown not going back far enough.

At some point, our sun will go supernova and you will end up drifting through space.
And all your life before that point will be less than a blink of an eye compared to the time that follows:
Trillions and trillions of years until the heat death of the universe.
And even that time will be less than the blink of an eye compared to the eternity afterwards, when you drift through a black void without any stars.

Forgetfulness. Think how forgetful people get after having lived a normal lifespan, now go for a few thousand+ years and you’ve probably forgotten whole centuries of your life. This is actually the premise of a solo journaling game Thousand Year Old Vampire, you have to cross out and forget memories as you progress through the game, just forgetting whole parts of your life.

Life will pound you into an uncaring jaded disinterested unloveable husk of a being after too many emotional scars from losing loved ones, too much of seeing humanity make the same mistakes, and too much watching the knowledge you gained turned irrelevant.

Or, life will beat into you an uncanny ability to converse and relate to others, even if fleetingly.

Watch The Man from Earth.

Basically all of the time you’re alive will be after the heat death of the universe, where you will be floating in space, with nothing to do, nothing to see, nothing to experience. Complete darkness, complete silence, in a complete vacuum, for eternity. Every other particle in the universe is forever out of your reach. You know that you will have nothing forever. You will never see, hear, or touch anything again, for all of time, which will never end. The trillions of years that preceded your float through the void fade into a distant memory as you outlive twice as much time, four times as much, a trillion-trillion times as much, and infinitely more.

Btw if you were actually immortal, after a while you would just go into shock and enter a vegetative state from all the psychological stress.

If you have epilepsy or Parkinson's or MS, you're just going to likely get worse forever.

Depends on the type of immorality. Do you continue to age? If no, what age do you stop? Eventually the universe will die. So what happens to you then?

It might be fun for a while. Maybe even a long while. But that fun will be gone in an instant compared to the trillions and trillions of years you will float in a dark dying universe of nothing.

If we're talking magical immortality, as in you can't die, at all. Then the fact that however much enjoyment and experiences you get while the universe still exist, it will be followed by an infinite stretch of nothing after the heat death of the universe.

Family meals that take 3 restaurants No retirement

Nobody is answering the prompt lol. Everyone says all of this shit all the time.

You live long enough to never feel at home. Sure the loneliness sucks or whatever, but who do you root for at the football game?

Having to buy new shoes for the rest of eternity. You know how much work I've literally just put into finding shoes that 1) don't suck and 2) aren't made with slave labor? It's impossible. Drives me insane. I'd found my own shoe company once I become immortal rich just to fix that problem alone. Maybe other stuff too we'll get there

I suppose on that note: it seems like a really bad idea to become a public figure after a while. Like you obviously don't want your immortality found out. You have to have like illuminati power before that point though, but it could happen at any time. Like if something happens and you become a news item (i.e. helping someone out and a video goes viral online). Not saying everyone is all that close to going viral, but over a sufficiently long lifespan you're effectively rolling that dice a lot.

That old person feeling of no longer being with "it", and what's "it" now being strange and scary probably compounds over the centuries.

immortality doesn't guarantee perpetual health, you're alive, but so broken and sick you wish you could die, but you can't

Discovering the upper limits to what the human mind can retain and just constantly forgetting all the shit you used to find important.

A lot of ways to die are excruciatingly painful, but you die, so you don't live with the pain. If you end up in one of those situations and don't die (because you are immortal), I imagine the psychological impact of the pain without immediate release could be enough to completely break you, mentally.

If it's just you being immortal, loss of all family, and friends, and loss of new friends, rinse repeat forever. Eventualdieyoull watch society collapse and regrow (possibly), and the planet will die. Immortality is forever after all. Then you're left alone on a deserted dead planet. Electronics you have will eventually break and fade away to time. The sun will grow and die off, and it'll burn because you're immortal but still stuck on a planet that'll get enveloped, eventually. Living forever would be terrible unless it was forever until you died of something physical, just not age and illness.

Time gets shorter. I've already experienced this going from my teens to my twenties and into my thirties. I can remember entire weeks of my childhood. By the time I was in my mid twenties, days and weeks blurred together. Now it's like months go by and I don't even notice.

People talk about it more as they get older. Eventually when you enter your 80s and 90s, it's like entire decades can come and go. So imagine when you're immortal. If you've been alive for 100,000 years, that's longer than writing has been around. Entire civilizations will have come and went.

But from your perspective, it's all a blur. Entire genealogies were experienced, yet those people barely registered in your mind. If you had a favorite food, maybe the recipe disappears when you went four centuries without eating it. Jokes and fashions you're familiar with are completely alien to everyone else. Are you even capable of noticing when things change at that point?

There's also the question of how human are you? Everything and everyone would seem inconsequential. Would you even be able to socialize with others, or would you be completely sociopathic? That's if you don't hurt anyone and get tossed in a jail cell. What happens if you spend a few centuries in prison? Fight in multiple wars? Would you even feel the slightest discomfort when you kill someone?

Knowing the answer to some of history's biggest mysteries, because you were there, but being unable to speak about them because, 1, that would expose you, 2, nobody would believe you either way because nobody expects you to be THAT old.

Also, it is already frustrating seeing kids being dismissive or denying events that you yourself have lived. Imagine being thousands of years old and seeing so much shit, but those events are rarely retold, forgotten, or straight up denied by conspiracies or future governments that won't admit their fault on it.

If the ultra rich find you out, you can expect lab-rat life, at least until all modern systems collapse. Death is the only thing those suckers fear, because regardless of their net worth, it comes for all, even if late. They would do anything to find out your trick

Given a long enough time frame, the vast majority of an immortal life would be spent buried beneath something or floating in the void of space. Think about it, you outlast planets and stars. When those go dark, but you don't die...nothing to do but float in space.

You might counter that with, "well yeah, but eventually I'd find other sentient life forms and/or people again.” And sure, maybe, but that wouldn't last as long as you...and then you're just alone floating in space again, for the vast majority of your life. The only thing to look forward to, since you will outlast everything, is the end of time itself.

If you're injured and you survive with the scarring from said injuries. Well, good luck because you're now going to wear those and wish you had died from them. If you're incapacitated or amputated? Gotta live with that for years and years.