I hypothesize this happened to me somewhat. I was just under the radar enough for the GATE (Gifted and Talented Education ) Program that I didn't get in. 4th grade through graduating High School was a breeze, even when my bad study habits caught up with me. I even got into university too!

But once there, I got a major kick in the ass. Classes were hard all of a sudden. They would fail you and leave you behind if you didn't catch up.

I also tend to flail when doing a thing, it goes well for a bit and then all of a sudden it gets hard. My formative years in school didn't really prep me for handling a challenge.