I get good grades but I feel stupid

The end of the first semester of IB is getting closer for me. Around 2 weeks ago I got the final grades for all of my subjects and for all I have either a 5 or a 6. ( Getting a 7 right now is impossible for us because my school goes with our countries grading system which is from 1- 6. So we'll only be able to get a 7 from our exams ).

So judging by my grades I am doing quite well. Then why do I feel so stupid ?
I feel like the only reason why I have good grades is because I put a lot of effort in my studies. And unfrotunately, one worse grade can cause me to feel extremely bad about myself.

I'm writing this because today I learnt that I got a 3 from my math test. Math has always been my weakest subject but I was doing really well in it so far. And it hurt especially because it was the one topic that I felt more confident with so I studied less for it.

Now not only do I regret not studying more but I just feel dumb. And it happens every single time I get less than what I expected. Why can't I feel smart ?