I agree with another comment that suggested just pausing more frequently when you read. There's also nothing stopping you from rereading the same passage over and over so you can process it better before you continue on.

It doesn't work for everyone, but I do recommend trying to read aloud and really listening to yourself talk. Especially for dialogue- I always try to read dialogue as I imagine the character would, in order to help me understand what's being said.

Paraphrasing is another strategy- if the book has complex/flowery language that makes the plot hard to follow, perhaps try taking each paragraph one at a time, figuring out what is happening, and reword it in your own words?

Online resources are good for understanding what's happening in the book, but don't let that be the only thing you do. If anything, maybe find a plot summary online and then read the book, so you'll at least have an understanding of what's happening story-wise to help you figure out other things like themes, characterization, etc.

I know this can feel discouraging if these strategies (or any others that've been suggested) make this work so much harder for you. I do recommend talking with your teacher(s) about this, if only to communicate to them how difficult assignments like these are for you and the steps you're taking to try to accomplish this. Most, I imagine, will be very understanding; the may not be able to change any assignment guidelines for you, but they will appreciate the fact that you're communicating with them about your struggles.

And you're not dumb, friend <3