Well no, that doesn’t make you dumb. You might be expecting too much out of yourself though. Are you reviewing the memory palaces? They work well to get something into long term memory right away, but that doesn’t mean permanent memory. Ever come back from summer break and have to relearn last year’s math?
Don’t be afraid to ask your teachers for advice on what parts of the book are necessary to study, and for samples of prior years’ tests to inform what you focus most on studying.
Check out Beau’s advice on a similar thread. Studying for a test with 1500 multiple choice questions in 12 days ;( - #2 by beau2am