Just like any science, mathematics has a solid structure. It can be compared to a multi-story building. If you have not built the first floor (that is, you have not mastered a specific part of it), then you will not be able to build the second floor (i.e. study it further). Once you miss some key point, then the whole system fails.
Mathematics requires a lot of diligence and perseverance. You need to learn the rules and formulas, carefully consider tasks. The slightest mistake leads can unmake all your prior mental efforts.
Another difficulty is that you need not only knowledge but also the ability to solve non-standard problems, that is, those that you have not yet met. Of course, many get confused at this point.
Some students have trouble memorizing formulas. Others can’t get a grasp of the problem and get lost in the middle of complex solutions. Also, demonstrating the process of solving problems can be a great challenge, as teachers usually want to see how students came to their solution step by step.