Take notes, generally write a lot e.g. by copying paragraphs or, even better, try to summarize the key point of a paragraph or page into a sentence or two. Use markers. Reading aloud as suggested by FroshKiller is perfect. Do crossword puzzles. And don't use regular screens for longer reading (paper is still best, e-ink kind of ok).

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.