The son of a family friend has just dropped out after the first year of his Medicine course. He's really smart, and had no difficulties with the intellectual demands per se. But it turned out that being very intelligent/quick on the uptake wasn't the critical factor.
A great deal of that first year was memorisation of anatomy and physiology, and there was a lot of opportunity for extra curricular activities. Because he wasn't *that* interested in the subject, he found it hard to put in the hours.
As an old git, I now know a lot of doctors both professionally and personally, and I can assure you that cleverness is not necessary to have a good career in Medicine! You *do* need to be interested and self disciplined. Oh, and having compassion helps!
Finally, like others people have suggested, "imposter syndrome" is really common - almost universal - for people who are high achievers. You're more like the cleverclogs you fear than you suspect!