Straight As throughout highschool because everything was so brain dead easy to me and i was always recognized as among the most brilliants by teachers and peers. Everyone expected greatness from me. Straight As throughout freshman year of college because I was still running on amassed knowledge from my time spent on the internet. My GPA started to drop hard 2nd and 3rd year and I was almost expelled twice from the University. I have always had trouble focusing but never looked into what could be a case of ADHD but it wasn't really the big problem. During these years I was taking the core requirement courses of my biochemistry major. As "interesting" as these courses are, I always felt like they were quite useless information as we learned very little about practical applications of these theories. As such, I wasn't interested in school at all and ended up skipping the bulk of my classes and procrastinating. During this time I was also losing my own philosophy on the purpose of learning so everything felt so worthless, including myself. Everyone around me were working so hard at memorizing things and doing well in school but this was all pointless to me as I felt that most would forget everything after their exams anyway. Good grades weren't enough for me to feel rewarding about working hard. 4th year I needed to put a lot of effort into saving my prospective career so I worked a bit harder but things were much easier as I was now taking electives like physiology, endocrinology, virology, immunology, etc... These courses were like extensions to the previous required courses and I started to feel their purpose. They were actually fun to learn and I felt good about knowing what I knew and I started getting good grades again. I am now back on track with a renewed interest, hoping that I haven't screwed up too hard to destroy my chances at medical school.
tl:dr It doesn't matter whether you are smart or not. If you cannot understand the necessity, purpose, and the joy that knowledge brings then your education will fail.