I said weather too, but added COMM radio and procedures. Amending that I think right now radio is the hardest. It mixes in the airport classes, terminology, many more acronyms..(I work in IT, and our system has tons of them as well, some the same as flight acronyms but with different meanings) and quite a lot of different rules depending on many factors.
I'm hoping that during flying lessons I get more adept and some of the things become automatic. There is a lot to learn in ground school. I really respect people that have made it through and learn all (or a great deal) of this. I am older (almost 60) but luckily have never stopped learning, but even so I always get amazed. When I was learning aerodynamics, everything was new. I had to memorize some formulas, but more important had to understand the concepts, and keep them straight in my mind. Some of the things show up again in learning about instruments and motor, mechanics of airplanes, and then...when I go back over aerodynamics some concepts that I used to struggle with are like "well, of course..." as if I knew it all along.
Same with weather. I had to learn about air density, effects of temperature, etc. in aerodynamics, but then they show up again in weather theory. It seems to me that learning for fly and about flight is not linear learning where you learn something, build upon it, build upon that, but instead it has some linear but at the same time seems like you need to learn about everything at the same time.
Kind of like if you had to write a dictionary from scratch. Well, you can't really have the first definition of the first word, because you already need words to describe the first word. The foundation is the thing.
I still am struggling with weather, and radio. Have not even gotten to navigation (except in small doses).