My answer: it's hard to pick one, but JD Salinger's Franny & Zooey would have to be it! I read it when I was pretty young. It was the kind of story that embodied my existential teenage thoughts and made them feel a little bit understandable. It didn't feel patronizing as a lot of YA books did, nor did it condone being angry and indignant all the time: Franny's side of the story was full of her relatable disappointment with the world, but Zooey's seemed to put those overblown ranty feelings into perspective. (I feel like Catcher in the Rye could have used more of that balance, despite how much I love Holden.) Have an angry quote from Franny: "It's everybody, I mean. Everything everybody does is so--I don't know--not wrong, or even mean, or even stupid, necessarily. But just so tiny and meaningless."
Honourable mention goes to the play How It Works by Daniel MacIvor, which will forever be my go-to inspirational story. Quote: "Everyone has it. The thing that keeps you up at night, or makes you not trust people, or stops love. The thing that hurts. And to stop it, to stop the hurt, you have to turn it into a story."