This is exactly what happened to me as a kid, but in my case it wasn't curse of the "gifted" but curse of the pushed. In my younger years growing up in Ukraine my parents and granparents pushed us hard to learn math, and so did my school. Endless repetition, hours of homework, Saturday classes, etc. I didn't have any sort of gift for math, this is just how children were taught there.

Moving to the US and going into 5th grade, the first several years were just repetition of things I'd already learned in Ukraine. They were so easy. My grandmother wasn't there anymore and to my parents it looked like I was doing _great_ so they also loosened up and didn't really push me further.

When we finally got to substantially new material I started to struggle. I got too used to everything being so easy and built on concepts I'd already had drilled into me. I failed a math test once - even my teacher was surprised. I ended up falling from perfect score As to Cs, then scraping back up to Bs by the end of high school.

Now I regret not taking math as seriously as I should have after leaving Ukraine, and am now taking time at home to learn on my own. Thankfully I'm used to self-learning in other areas, so I think it's going pretty OK.