This part in the original comment resonated with my experience: "When I was a kid, I could grasp concepts easily and I understood maths very quickly so I didn't really bother doing things like homework or actually studying."

This was exactly the case for me after moving from Ukraine. Concepts were easy because I'd already known them, or they were just starting to be built on top of stuff I'd already drilled and knew before. So I did not put any effort into homework or studying. When entirely new concepts came along they crept up on me and my bad habits and I failed hard.