Calculus is not easy. It builds on a lot of the Mathematics that one has learned over the course of one's life.
And sadly, we often flush what we learn not long after we learn it! But Calculus is wonderful!
The discovery of Calculus enabled lots of non-genius folks like myself to make a wide variety of very useful calculations that before required a genius to make.
I would also venture that to truly understand Calculus helps one to understand a wide range of important phenomenon with less effort. Unfortunately, though AP Calculus often leaves students without a true understanding of the subject.
I know this very well since my father often teaches College Calculus and has to reckon with students who arrive with AP Calculus credit but who find that college-level Calculus is a lot more challenging(and meaningful) than the rote repetitive tasks over-emphasized in AP Calculus.
This is why I think it's a worthwhile investment for high school parents/students to hire a tutor for their daughter/son while they are taking AP Calculus. I've also discovered that sometimes a good math student may struggle with AP Calculus because they missed some of the initial concepts that are then built upon during the rest of the course.
I've been able to help students improve considerably after only one or two hours of tutoring.
This is because for a skilled tutor, it often only takes a brief discussion to identify what they're missing in the foundations of Calculus.
I look forward to helping more AP Calculus (or Statistics) students in the future.