Even since preschool i could do math in my head beyond my grade(3 years ahead according to psych tests that we all had to undertake), and all of that was killed by a single teacher who had me under her care for 3 years.
She just plainly didn't believe that 10 year old child could solve all exercises in head. and forced me to write down each single step - while the exercises themselves were already too easy.
I never got into a habit of challenging myself, thanks to her it is quite the opposite, and i now am really bad at doing math in my head.
I would say that lack of challenge was the biggest flaw in my education, i went throughout whole education(including university) without studying at home - not because i was so damn good, just because in my formative years the habit of working hard was stripped from me, and C-equlivalent to B-Equivalent was good enough for most subjects.
I expected for that to change as i managed to get into top 3 high school for my area.. yet it was mostly the same.
I am slowly regaining it thankfully, but it is way, way harder to do so as an adult. And i still wonder where else i could be in life if not for that single person, and what happened to other such children in this post communist country.