Spoiler: >!Everything is fictional. Nothing is real. You are the only being who exists at all, and everything that happens/has happened/will happen is all in your mind. At least, that's one philosophical concept of the universe.!<

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While it is true that fictional works are tales of events that didn't actually happen, the same is probably true with at least parts of memoirs and autobiographies.

And really, does it matter if it happened? If you read real, true, completely accurate accounts of battles, or journeys, or other events that occurred thousands, or even hundreds of years ago (or during your lifetime, some distance from you) could you really tell if it was fact or fiction? Would the story be more or less impactful or meaningful based your belief the events occurred as described?

Another point to ponder: there is truth in fiction. Or at least, that's what many (most?) authors strive for: expressing truth through storytelling. For instance, most people will probably agree that doing the right thing is important and good, even (or especially) when it's hard to do. A common theme in HP, as I'm sure you know, and many other stories. And of course, there are many other true things expressed in fiction. So despite people and events being imaginary, there is still a lot we can learn. In fact, reading is a pretty good simulator for real experiences -- research with fMRI (if I recall correctly) shows that the same areas of the brain are active when reading about something as when experiencing it.

And as /u/Petal_Chatoyance said, suspension of disbelief is powerful and important. Try to nurture it, and re-develop it if necessary. I'm not sure how you do that -- maybe just tell your brain to chill when you're trying to enjoy some fiction, and it starts mumbling that this isn't important because it didn't happen (in our reality). Or maybe take a break from fiction and enjoy some non-fiction for a while. Or try some less speculative fiction (mysteries, thrillers, adventure... things set in our world, or at least adhere to the same limits of technology, physics, biology, etc....) It may be easier to suspend disbelief is there is nothing too fantastical about the story. Maybe try things like that until you start craving something more fun or escapist.