Thanks for the reference.
In college, I averaged one math course per semester.
In most of them, I sweated blood to get a “C”.
That’s the thing about math. Almost everything depends on a mastery of prerequisite material, and once you take a course and fail to absolutely master it, everything that follows and depends on it is going to be a struggle.
I guarantee that if you were to start over at ground zero, say elementary algebra, and concentrate on studying as hard as needed to get straight As in every course, you’d get them no matter how far you went in mathematics. All the way to a PhD, if you wanted.
I’m convinced that anyone without brain damage and possessing moderate intelligence and an unbreakable will, who has the work ethic and refuses to allow himself to become so discouraged that he gives up, can go all the way to a PhD in math, getting top grades all the way. There are a lot of people with 120 IQs walking around with their doctorates in math.