Some subjects feel like they require more intelligence. If understanding an idea requires you to take different ideas into account, it may be more challenging to keep track with a lower working memory. e.g. mathematical proofs.

Conversely, if a subject requires a large volume of memory, but each fact or idea is separate, it may require lots of practice and less working memory. E.g. history or law.

Because we can increase our working memory within a subject, we can create new chunks and master very complicated ideas. The perceived difficulty is often the way subjects are taught. Only the smarter students will remain, the higher you go in maths or science. This leads teachers to skip over "trivial" steps, leading you feeling like you can't master it.

Solutions are for teachers to slow down the class or for the student to slow down the class using, for example, the Feynman Technique.

Any subject can be learned with some practice and patience. Some subjects may appear harder because of the speed at which they are taught.

To fix this problem, you may need to put in more work. If you can't see everything at once, it just means you need to practice more of the pieces, so your mind does not have to juggle so many ideas simultaneously.