All reading is valid, and fan fiction is not inherently worse than any other of kind of reading. Just like any other genre, there’s good and bad writing to be found.

That said, you can definitely find books that have similar themes to fan fic. For example, if you like slash, there are plenty of m/m romances to found. That’s true for most other themes as well. Ebooks that you can read on your tablet or phone might also help you vary your reading. If you have a library card, see if your library offers Libby. You can also look on Amazon for smaller press or self published works that might fit your interests. There are plenty of strategies that can help you switch up your reading habits, but, I repeat, reading fan fiction is still reading.

Edited to add…

Fan fiction is often a kind of comfort reading. Because the characters and settings are familiar (even in an au or crossover), there’s not the work of understanding a new world or figuring things out that you get with a new novel. It’s comforting and easy for some people to revisit characters that they know they like. If that’s true for you, trying to find books similar to your favorite fan fictions might be a good approach.