I am not in the field of mathematical research (yet), but I believe that an experience that I have recently had might provide some insight on your problem.
I once attended a pure maths summer program for high school students at a university. Just like college students, we slept in dorm rooms with roommates, used the dining hall, and were allowed to go out into town with pretty minimal restrictions and a lenient curfew.
The mathematical curriculum of the program was extremely rigorous, and to complete the homework assigned each day, one had to work on it for the entire day (and sometimes this was not even enough time to finish). As someone who really loves pure and recreational mathematics, I really enjoyed this - math is my "bread and butter," and it takes a long time for me to get tired of it (though I am by no means gifted with inherent mathematical talent).
However, a lot of the other students at the program did not get as much out of the program as I did, even though most of them loved math as much as myself, and were even much more brilliant and talented than I. I think that this was in large part due to the fact that many of them were having a hard time adapting to the "college lifestyle," or even abusing their newfound freedoms. For example:
Many of them spent much of their time wandering the city with their friends, getting caught up in petty social dramas and romances, or just goofing around.
Lots of people couldn't feed themselves properly, and lived mostly on junk food.
Some people seemed like they just didn't enjoy math that much, so they avoided the homework and looked for other distractions. This made me question why they had even applied to the program in the first place.
A couple of people suffered from depression having to do with family or personal issues, but this was not so widespread.
The biggest problem, in my opinion, was sleep - people often procrastinated on the homework all day long and then stayed up as late as $4$ in the morning on a regular basis to finish it up in a hurry. As the program progressed, people suffered from chronic sleep loss (including the administrators of the program), and fell asleep during lectures, during the middle of the day in public places, etc. Then, when it came time to do homework, it seemed like they couldn't think straight.
My point is this: your "intelligence" (whatever that means - it's not really a well-defined term) isn't the only thing that has a bearing on your academic performance; in fact, I would argue that it is actually a minor factor. Health, emotional state, focus, and motivation are probably the most important.
When I was at said summer program, I had to buy a sleep mask to help me go to sleep at night because of the bright city lights in my window. I also had to keep an eye on my diet while I was there to make sure that I didn't eat too extravagantly. I urge you to do the same, or seek other solutions, for whatever personal problems you may have.
It seems like you genuinely love mathematics like I do. No matter how "intelligent" you are, if you have the motivation to keep studying math and you can control your lifestyle, I think that you can succeed (however, choosing your next set of classes more conservatively may be a good idea).