The main reason this is hard, is that learning to write proofs that are both rigorous and human-readable is extremely difficult without a teacher+grader.

This process is not taught in high school, most or all textbooks implicitly assume that you know how to do it, and if you don’t know how to do it then it isn’t obvious what’s wrong until you realize that you can’t do the exercises anymore.

Math is largely proof-writing. Proofs are an interactive process between writer and reader. Without a reader there is no feedback loop and you don’t learn to write understandable proofs. If you can’t explain math than you don’t truly understand it.

Once you know how to write proofs, it becomes possible to learn more through books. But you really need a class setting for that first part.