I might not have been the best at math but I've sometimes been considered a "math whiz" - I took the undergraduate math seminar at UCLA when I was a High School senior.

I've always had the impression that what made people bad at math is the exactly the "grind" attitude - "focusing" on a problem only reduces your creativity. In fact, whenever I took this attitude, I became bad too.

Playing with a given problem every way you can is good. Enjoying a problem and finding it interesting is important. Grasping the concepts is good. Letting a given problem go whenever you can't solve it is good - I think there's a place below conscious awareness where problem solving can happen well.

But don't just grind on mathematics, that's poisonous.