Being “smart” made school easy for a while. Then the easy part ran out and the work habits never showed up.

Coasting on quick understanding works right up to the level where everybody understands quickly. What separates people after that is study habits, and those get built by struggling with things, which is precisely what an easy start protects you from. Hitting that wall feels like losing your ability. Nothing was lost, the work simply started asking for something you never needed before.

If you are there now, treat it as a missing skill rather than a verdict on you. Learn how to study properly, badly at first. Everybody who arrived earlier had to do the same thing.