If every story has already been told, the story itself doesn't matter; what matters is how you tell it.
Now, I'm the first person to try to defend the idea that there are still new stories to be told. Maybe it's egotism that makes me crave a unique, 'edgy' idea (and my life's goal of writing a banned book). Maybe it's just a trait of all storytellers to want to be the one with that brand new idea. But at the same time, I also believe in the idea of the Hero's Journey, and that every story is, at the root of it, basically a story of growth and learning. Characters, setting, genre--it's all just window dressing.
But that window dressing is the stuff that we as artists and creators can change. It's the stuff we can twist into new shapes and reorganize into fresh concepts. It's the stuff that sells books, when it comes down to it.
And when those details are 'copied' by someone you've never spoken to, or pop up in a television show created halfway around the world by people way more famous than you--that's when the ego takes a bruising. You're forced to entertain the idea that your wonderful, sparkly idea maybe isn't as unique as you thought it was. That can be painful and raw, but it's a fact of life.
Our task, as writers and artists and creators, is to not let that fact get us down. Keep creating, writing, chasing our art, no matter what other people out there are doing.
So what if your story includes Loki as a CEO, like this manuscript that just got sold? So what if you're writing an epic fantasy with a Sealed Evil In a Can and a Reluctant Warrior hero?
Just keep writing. Keep making art. Keep finding new things to explore and twist and smash together to make your project your unique and representative of your imagination. It's a hard lesson to learn, and one that will likely require plenty of re-learning every time it crops up. But I think it'll make us all better people (and artists) in the end.