There's nothing wrong with having a hard time understanding books you're reading. Don't fight against your skill level, accept it and work on it. I personally read very well, but I have had family members who have struggled due to the kind of issues (dyslexia etc) people are suggesting you might have. The best thing isn't to battle against that, but to accept what works and what doesn't for you, and build on it.

Importantly, nothing you do that works for you is wrong. Nobody is going to die or get hurt because you take notes about a book, or read in short bursts, or need quiet to read. If it works for you, it's great. If you need to stop and think, do it. Don't think this is wrong or bad of you. It's not, it's how you need to understand things. I've read books before that I've needed to put down and read later in life to understand. Everyone has their own pace.

Lastly, please have confidence in yourself. You can do it, whether it takes long or short.