It’s very easy to believe that the person that wrote your textbook is a supreme authority on the subject and if you can’t understand their explanations then it’s also easy to believe that you’re to blame. You’re not.
Not everyone ‘gets’ things in the same way. People need things explained in a way that they can relate to, and we don’t all have the same set of references. Authors make assumptions about their readers’ level of understanding that aren’t necessarily true and some text books are just incredibly dull.
If you’re struggling to make sense of a topic, do yourself a favour and find a fresh perspective, you may be pleasantly surprised.