every time I read textbooks I find things that I did not know as well as I think I should. It’s almost like having layered questions: you only know the second set of questions to ask once you have acquired a certain understanding of the basics. And you can only hope that this never goes away because it is a sign of growth. That way you get deeper and deeper into any material.
ACSM exams are not easy to pass but they are not impossible either. You have set the bar very high for yourself by selecting this organization. Once you have studied all material and feel grounded in it, go ahead and take the exam. I am sure you will do well.
Best of luck!