I think everyone zones out to some degree, people just do it to different levels. I understand your post though, because I used to get anxiety over the same thing and it made me not want to read at all. It goes away with time when you realize what sentences or paragraphs are more important than others. Don't be afraid to read slow, but also don't be afraid to skim over some sentences. You won't remember every sentence anyway, the important part is understanding the plot and for the reading to be enjoyable.