You live in a simulation, anon.
You may or may not have entered it of your own accord. Perhaps we solved all the important problems "up there", and it is very boring. Perhaps because you are bestowed with a prize if you play properly.
Fundamentally, this is a game, anon.
> Isn't this a very nihilistic way to look at life? Does this mean that our actions here fundamentally don't matter?
No, the scary part is that EVERY SINGLE ONE of your actions matters. Maybe this is a game show and we are competing; perhaps we exist to entertain our immediate masters, much like slave gladiators entertained the Romans. Did their actions in the colosseum not matter? No, they did, because their performance in the gladiator world determined their freedom and quality of life in the Roman world.
> I don't believe it.
Then don't, you're only shooting yourself in the foot. I wish you good luck nonetheless.
> Ok, I believe it. What is to be done? How should I live my life?
Aim at the greatest possible good. That's how you win the game.
> What is the greatest possible good?
The one that maximizes utility (and/or happiness) for the greatest number of players. Actions that decrease other players utility (and/or happiness; e.g. "sinful" behaviour directed at others) decrease your score, and incur dire consequences after the game ends.

Good luck, anon.