I was "good at math" in high school, and then when I got to my sophomore year of college I suddenly felt like my ability was all an illusion. Vector calculus, differential equations, complex exponentials... junior year saw quantum physics and complex analysis and I struggled, a lot. I definitely avoided more strictly math classes because of that, until I started graduate school at UCLA and realized the math is what I liked best! After working hard at it for a few years, I was back on track. At age 29 I took analysis again-- and this time I didn't stop until I was part way through advanced graduate classes, and I only stopped because I needed to focus on my dissertation.
If I hadn't happened upon mathematics again, where would I be today? It's so obvious to me now that when I was 20 years old, I simply didn't have the preparation of the other students (most in the complex analysis class were math majors, for example).
So if you once liked math but at some point felt like you just didn't have the ability of others, I hope you read this and think again. We will probably not become Terrance Tao's or Elon Lindenstrauss's (two recent fields medalists, the second the son of the mathematician for whom the Johnson-Lindenstrauss lemma is named). But with a little work we can be a lot more comfortable with math that we used to be.