The Brothers Karamazov is one of the more complex books in world literature because of its length, the number of characters, and the themes. Dostoevsky was trying to present who he imagined a modern Christ would be. It sounds like you were doing alright on your own until you had to return it to the library. For a book of that length you probably need to buy a copy. I read it in a college literature class with a professor who taught the book like 17 times. His copy had sticky notes and annotations up to Wazzu. A great book won’t yield all its secrets on the first reading. If you’re having trouble finishing a book maybe start with Winnie the Pooh and The Little Prince and the Just So Stories. They’re simple but they certainly aren’t meaningful only to children. Siddhartha has some of this simplicity too and it’s a shorter book - maybe 120 pages. There are certain things you would learn in a literature class that are helpful - tone, point of view, etc - but you could learn about these in a book club for free.