oh interesting with formative fiction... there are some books where, the way I describe it in my head, there is a "before" and "after" reading them. Like, there is a bit in The Dark Is Rising (uh, one of my formative books!) where Will thinks, "the world he had inhabited since he was born seemed to whirl and break and come down again in a pattern that was not the same as before," and that's how I feel about these books. A Wrinkle in Time, The Dark Is Rising did that to me as an elementary school kid... Ender's Game (I know, but it did) and The Tombs of Atuan to a somewhat lesser extent as a middle schooler.

interestingly, I can't think of movies or TV shows where there was this sort of stark "this changed the way I thought about things" except for, of all things, Star Trek V, which was the first Star Trek I'd ever seen and which I was like, "wait, you can do this??" Which is highly ironic because I of course learned later that most people don't consider ST:V to be a particularly good movie, but it blew my mind.