I studied English literature at university and I used to have this same issue and I think it was largely caused by the pressure of feeling like I had to complete and understand the text in a short time. I would treat books like a puzzle, I’d be half writing the essay in my head as I read it, would focus on key words without reading the sentences.

The best advice I could give you is to step back and give yourself plenty of time. Read the text like a book and try and divorce it from being a ‘school assignment’ if, by the end, you can’t make sense of it in the context of the essay you have to write/questions you have to answer you can go back an reread sections or the whole thing. You’ll find it easier for having already known the story, plot and charactersz