My math was ruined because no teacher ever said this one thing to me: "Arithmetic is about getting the right answers and mathematics is about learning the methods" or something like that. Had I understood that, I would have applied myself to the correct goal. As it was I still thought the goal was to come up with the right answers, so all through High School I gave them the right answers and they always said "Show your work" and I'd say "I did it in my head" but I couldn't write it out the way they wanted (and I thought they were stupid for not giving me credit when the answer was correct.)
One sentence would have changed everything, and no one ever told me.