When I was a kid / teenager, on the other hand, I could read anything fictions or non-fiction, technical or scientific cover to cover if I wanted, and even maybe get somethingout of it.
Read a lot (and writing!) even if "nothing sticks". It sure will feel frustrating, but don't worry, it will improve again over time "simply" with practice. Don't feel abnormal, all this is quite ordinary as you grow up and more and more "stuff to think about" accumulates. It takes practice to get to a brain-relaxed state again. Mindfulness, meditation etc of course can help a lot too.
I sometimes won't be able to write, not because I have nothing to say, but I associate writing with pain (I have bilateral tendinitis).
Similarly, you may have eyestrain but not be conscious of it.