I know that this is my personal view on life, but:
If you don't care at all about what happens after you're dead, what's the purpose of your life?
You'll be dead and it won't have mattered a bit that you're been here.
If you're strongly moved, you could commit your life to doing something about it. But it's also important to keep in mind that none of this likely matters long term, on the scale of millenia. Future historians will likely consider these times as far more tragic than the fall of Rome. But that's how it goes.
In my case, not exactly nihilism, but est/Landmark. Basically, there is no innate meaning or purpose, only whatever you say that there is. As a biologist, of course, I'd add that there's selection for reproductive fitness. But that's readily ignorable, if you're paying attention.
Maybe from a selfish and individualistic point of view (which is one of the reason things are going in the wrong direction), but if you have empathy and care about others people and life forms it is a very different matter.
I feel strange saying this, but really, it's not going to be that bad. Sure, human civilization may collapse. Billions will die. And we're pretty much locked into an historic extinction event.
But the human race won't get wiped out. And the global ecosystem will recover. It's not like Earth will flip into Venus mode. In a million years or two, this will all be just a blip.
Having fun is the best I've come up with. And yes, working on stuff that I care about. But that's part of what I consider having fun. Long term, I'll be dead, no matter what.
I don’t think it matters as much if what you do in actuality ends up making a significant difference. What matters is that you did what you could. From the perspectives of future societies, you and I will be blips anyways, so that’s irrelevant. But how can one be contempt with knowing that they didn’t try to make a difference, and that their mark on the world disappears when the energy has left their body and their corpse has been burned and buried, I do not understand, because if you don’t (try to) leave anything of value behind, your existence is practically for nothing, except for maybe even a net negative.
This removes anxiety about fulfilling your purpose.
Nihilism can be a source of great comfort when practised by optimist. :)