This is funny, but even the most intelligent people are inflicted with this. Don't let it keep you down, we cannot be good at everything.
Its been consistently self-reported by Harvard students. And another effect is present, too - excellence leads to being placed in competitive environments, where everyone else is just as excellent. And this can make brilliant people feel stupid.
Intelligence also doesn't necessarily translate to actual success. I've been through numerous assessments as a child that confirmed I am comfortably in the "green zone" (if measured by IQ, that is), but I also have pretty severe ADHD so I can only really make use of my brain for short periods of time.
I can get a week's worth of work done in a day, but only once a week, and I spend the rest of the week wondering where I'd be if only I could work like that every day. I was also a decent student in school/uni but never near the top of the class, because I couldn't bring myself to study for anything more than a few days before the exam.
Well, there necessarily need to also exist below average Harvard students. Its probably more of a shock to have the one thing you might have been proud of, being rather smart, be taken away once you get there and realize you are probably at most average and have to find a new identity