Books are not scrolls. Scrolls must be read like the Torah from one end to the other. Books are random access --a great innovation over scrolls. Make use of this innovation ! Do not feel obliged to read a book from beginning to end. Permit yourself to open a book and start reading from anywhere. In the case of Maths or Physics or anything especially hard,try to find something anything that you can understand.Read what you can. Write in the margins. Next time you come back to that books, you'll be able to read more. You can gradually learn extraordinarily hard things this way.