>>105840544>>105840698People who struggle in math courses usually lack:
1. Algebra skills. People tend to look down on the term "algebra". But weaker students who struggle in proof-based calculus, analysis, and other courses literally can't do the necessary "algebraic manipulation". AOPS' prealgebra and intro to algebra books should be enough.
2. Problem-solving skills. This is something you train slowly by solving (attempting) many problems. Techniques (heuristics) as taught by books from Polya, Larson, Zeitz are only part of the equation. There is no shortcut.
3. Proof-writing skills. Again, people tend to look down on this. Many even say you should not read books that teach proof writing. Those people may not need this, but if you're struggling with real math courses, chances are you also lack proof writing knowledge and experience. Grind Hammack's Book of Proof. It's free and good.
Feel free to ignore the advice above. In most cases, if you keep doing problems, you will improve in those 3 aspects anyways. IF you don't give up and then stop giving a fuck. I was just suggesting a more systematic, and less painful approach.