More like, you have a few months or days in your entire life that you are actually living, but you are alive for long, long after.

You can potentially live until the heat death of the universe. Nothing so special about your body, that not so distant future science would not be able to repair, recycle, or modify. It's usually your DNA getting so damaged, that the proteins you need aren't produced in as much quantity as needed. Eventually, you lose the careful equilibrium that you had in your youth, grow weaker, and die.

It's not such a big secret as it seems, it's just that there's a fuckton of the proteosphere, and it's really fucking hard to untangle all of it, or modify it safely.

The real issue is the moments that pass us by forever, the loss of meaning, and loss of status that come inevitably, and the loss of innocent wonder as well. Those initial moments DO cement who and what you are forever.

So the most messed up thing our sick society does, is convince young people that, because their projected lifespan might be something like 80 year old, that they have 80 years to live.

No, how much you have to live depends on you, what you live for, how you live it, etc.

Accomplished people essentially live their full lifespans.

While cubicle zombies are just waiting for the day they die, all meaning deleted from their lives (except their families, if they have any).