Somehow I found linear algebra easier than calculus, but I don't know why. I did both at the same time in university, but failed calculus 3 times and aced linear algebra at the first try. I'd expect being either good or bad at math, not both at the same time
Quality of teaching might have something to do with it.
But, also, calculus is much harder to understand at a rigorous, formal level than at an informal level.
On one level you can try to understand what the main concepts are about, be able to compute derivatives and integrals, solve optimization and related rates problems, and so on. I'd recommend Silvanus Thompson's Calculus Made Easy over any mainstream calculus book for this. In my opinion, the book succeeds amazingly at fulfilling the promise of its title.
But suppose you really try to read any mainstream calculus book, and understand everything. For example:
- Why are limits defined the way they are (with epsilons and deltas)?
- The book will probably touch lightly upon the Mean Value Theorem -- why is this important? What's the point?
- Why is the chain rule true? It reads dy/dx = (dy/du) (du/dx). Yay! This is just cancelling fractions, right? Any "respectable" calculus book will insist that it's not, but most students will cheerfully ignore this, still get correct answers to the homework problems, and sleep fine at night.
- Consider the function e^x. How is it defined? The informal way is to say e = 2.71828... and we define exponents "as usual". Most students are perfectly happy with this. But does this really make sense if x is irrational? Your calculus book might bend over backwards to define everything properly (e^x is the inverse to ln(x), which is defined as a definite integral), and it takes a lot of work to appreciate why.
In my experience, these sorts of issues mostly don't pop up in linear algebra, where the proofs tend to parallel the handwavy heuristics. I wonder if this had anything to do with your experience?