The world you were born into no longer exists.

The person you were born as no longer exists, at some point "you" died and you didn't notice.
- Beat me to it
  
  Sadly, the truth is, humanity is doomed. Within just 10 years, it will all look much much less welcoming. Much of the equator will become uninhabitable within a few decades. Horror scenario. Do not have kids. EVER.
- Good. Every single version of me deserves to die.
- Let’s be real here. Everybody knows you die every time you go unconscious. We just pretend we don’t know that because otherwise who would want to go to sleep?

The world you live in does not even exist. There might be one objective reality, but none of us where ever there. Everything you experience is filtered and distorted by your past experiences and current beliefs.

Not only that. You no longer exist. As you were at birth, that is. And after some amount of time every so often in your life, you will continue to be unrecognizable to the version of you from x years in the past. Your cells die off and new cells are created until you are literally composed of different matter than you once were. The closest thing to a constant thing tying you together would be the electrical signals in your brain. Memories. Like computer code sent from one PC to another. Also the DNA determining how your body is built.
It's the classic Ship of Thesseus problem. If you replace a ship's parts one piece at a time over many years until the old parts of the ship no longer remain, is it still the same ship? And in the same way, are you still the same you? Maybe our lives are full of many different people tied together only by our thoughts, memories, and genetic code.
- The neurons you're born with stay with you for the most part. Most of their complex organization is formed through a series of one time events early in development that can't really be replicated and then stays with you for the rest of your life. You get shingles when you're older because the same neurons were with you that got infected by chicken pox when you were younger are still there. There's a few limited areas in the nervous system where new neurons might be formed, but in general neurons are life long cells so be nice to your nervous system. Most other cell types in your body are turning over as you said, including glia and other types of cells in your brain.
  
  Username checks out
- In the UK we call this the "Trigger's broom" problem: https://www.youtube.com/watch?v=56yN2zHtofM

The phrase of something being a lifetime ago exists for a reason.
You're changing mentally and physically all the time, especially when you're younger and going through so many phases of growing up.
String enough of those changes together and at some point you will effectively become a new person, only connected to the previous iterations by memories that you share.
The death of one interation and the beginning of another is bound to be a blurry mess, but you can see it clearly over a large enough span of time.
The same is true with the environment around you, and even the wider world. It goes through iterations just the same as we do because nothing stands still, even if we want it to.

I remember being very concerned about Y2K and quicksand.

At some point in the future, everyone who is currently alive, will be dead and the earth will be populated with people who do not exist yet.
Since I was born when my parent were in their late 30's. And my father was as well, so my paternal grandfather was born in the 1890's. When I was a kid he told me stories of his childhood. I was only 9 when he died, and at the time his stories were just that; stories. Now (I'm in my 50's) I wish I had asked him things about those times. He was born and grew up in a world (mostly) without electricity, and without cars, phones, etc.
He was someone from a totally different world, and I knew him.
Added later: He was born "only" 129 years ago, everyone alive then, is now dead.

Thanks for the Depresso ☕️
- The best part of waking up, is depression in your cup 3

Honestly I think this is pretty well understood on an individual level.
The business world needs the wake-up call. It's an employee market, has been for awhile and projected to stay that way as employment remains high. Start shelling out some hefty salaries or people are going to move to those who will.

🍂🎶Just around the river beeeeeeend🎶🍂🦝

I got that when I thought about my grandmother. Born when the Nazis just took power, but Hindenburg was still alive. Grew up in Nazi Germany, fled her home as the Soviets approached and a lot of people she knew died in the bombing of Dresden, then worked in after war Germany without being able to finish school, continued through the GDR, saw the wall fall and Germany united again, participated in the introduction of the Euro.
What the world was, what Germany was changed a lot during just her lifetime, despite most of the time staying at the same spot.
Like, it#s not Balkan level, but it is pretty significant how much can happen in just one lifetime and how things are everchanging.
- Makes me think of that series Heimat. In the beginning their minds are being blown by radio; by the end they're flying on commercial jets.

"No man ever steps in the same river twice, for it's not the same river and he's not the same man"

The world of a second ago no longer exists either.
- There is no such thing as now. The line where the 'now' exists is between the future and the past. And that line has no thickness. Are we really alive if our entire perception is only of events that have happened in the past?
  
  It's the opposite: the past doesn't exist anymore and the future is only imagined. All there is is the now.

The phrase of something being a lifetime ago exists for a reason.
You're changing mentally and physically all the time, especially when you're younger and going through so many phases of growing up.
String enough of those changes together and at some point you will effectively become a new person, only connected to the previous iterations by memories that you share.
The death of one interation and the beginning of another is bound to be a blurry mess, but you can see it clearly over a large enough span of time.
The same is true with the environment around you, and even the wider world. It goes through iterations just the same as we do because nothing stands still, even if we want it to.

Anicca!

90's me was an absolute badass. Take my word for it.
Modern me... Eh, they're OK.

Believe me I know.