Read slower. Pause to think about what each sentence and word means. The only thing that's changed since you were 13, really, is that you've become too busy and performance-oriented (subconsciously rushing to get to the end).
Some weed and an afternoon of doing literally nothing can help unblock this part of yourself again.
I’m still chasing that high, and I feel caffeine gets in the way of both dreaming and imagining novel worlds, two activities that are a big part of my memories about my teenage years. Easier is to play a videogame, but even though so much is done for me there’s no guarantee I’ll meaningfully advance the story like will happen in a good book. I might still be immersed, though, especially with Minecraft/Minetest, Valheim, Factorio/Mindustry, and others. I am most thankful to myself when I opted to read, though; the benefits last longer and provide more experience to draw from.