When you read a textbook, you are reading a book written for you, or at least for some hypothetical version of yourself. Authors and publishers have tested the book in college classroom situations. The result, although sometimes bland and unchallenging, should be a text that you can read without undue difficulty. They are not written for you .
Authors write for audiences.
Documents result from a process of interaction between writers and readers, and to some extent reflect not only their common culture but also their desires, goals, and expectations. When you are reading sources written in past eras, you are out of this loop. Therefore you will find these sources difficult. Begin by recognizing the difficulties. Even when people talk to us face to face, we cannot always understand their messages and motives.