Cognitive load
Cognitive load is how much your working memory is holding at one moment. Your working memory can only juggle a few things at once, so when you pile on too much, nothing sticks. The fix is to break hard material into smaller pieces.
Your working memory is the small mental space where you hold whatever you're thinking about right now. It only fits a few things at a time. When new material asks you to track more than that, you hit a wall. The page stops making sense and you have to read it again.
That overload is cognitive load. Some of it comes from the topic being genuinely hard. Some of it comes from a messy setup, like a cluttered slide or jumping between five tabs. You can't make a hard topic easy, but you can cut the clutter and take the topic one chunk at a time.
So when something feels like too much, that's a signal, not a flaw in you. Slow down. Learn one piece, get it solid, then add the next. Small steps beat trying to hold the whole thing in your head at once.
Mara opened her organic chemistry chapter and felt lost on page one, with ten reactions blurring together. Instead of pushing through, she covered the rest and learned one reaction at a time. By the end she actually remembered them, because she stopped asking her brain to hold all ten at once.