Adding onto everyone's suggestions, my friend just told me this method and it really helps.
if you're finding it difficult to actually start re- reading, try this method, it works- really.
1. So... basically rather than starting from the start (like Frankenstein ... Walton's letters really make me fall asleep) maybe pick out a chapter/ section from the book you enjoy, or you think is really worth reading again. (you can start from the start too, your call fam)
2. So get a highlight, or a pen and read a chapter full, or read for a complete 30mins, highlighting and underlining.
3. Open a word document, TYPE (not copy and paste from a pdf) the quotes and add your own comments. Remember that you don't have to add comments to every quote, that's just a waste of time, add to the ones that you think is worth commenting on b/cause its deep. The others may fall under the obvious categories and there's really no point adding long comments to those, or you can one word labels for themes and whatnot.
This really helps you learn the quotes and actively think about the text.
4. you can do this at another time, like when you're on a study break but organise your quotes for each chapter according to themes/ characters and it helps you organise it in your mind too.
I really like this method b/c I used to analyse every word of the text, and it just wasn't efficient enough.
Hope it helps