I was doing my usual daily catch up with news, random stuff on the internet, etc when I came across this interesting comment. It's basically a guy giving some advice to a current MIT student. It reads:-

Alright, sorry about the delay. I was too busy celebrating the New Year. ;) I hope you're still checking in on this account.
Anyway, I think I have a bit of a unique perspective. I've seen MIT admissions from the perspective of the applicant, a student, a teacher, and now as an alumnus conducting interviews of prospective students. The fact that you mentioned MIT specifically really made me feel like I should take the time to produce a good response!
I wanted to start by writing out standard admissions advice (e.g. no one thing like SAT scores will keep you from being admitted, etc.). While all that is true, the problem you're dealing with is so much bigger than that. The problem you're coming up against is one I've seen so many of my fellow students encounter. If I could set up a wavy-fade flashback, I'd show you my freshman year.
I moved into one of the dorms at MIT thinking I was hot shit. I had, after all, just gotten into MIT. And beyond that, I had tested out of the freshman calculus and physics classes, meaning that I was able to start math "a year" ahead in differential equations and start with the advanced version of the physics 2 class we have. Registration went by easy enough and I was pleased with my decisions.
Term rolled in and I was getting crushed. I wasn't the greatest student in high school, and whenever I got poor grades I would explain them away by saying I just didn't care or I was too busy or too unmotivated or (more often than not) just cared about something else. It didn't help that I had good test performance which fed my ego and let me think I was smarter than everyone else, just relatively unmotivated. I had grossly underestimated MIT, and was left feeling so dumb.
I had the fortune of living next to a bright guy, R. R. was an advanced student, to say the least. He was a sophomore, but was already taking the most advanced graduate math classes. He came into MIT and tested out of calculus, multivariable calculus, differential equations, linear algebra, real analysis (notoriously the most difficult math class at MIT), and a slew of other math courses. And to top it all off, he was attractive, engaging, sociable, and generally had no faults that would make him mortal.
I suffered through half a semester of differential equations before my pride let me go to R. for help. And sure enough, he took my textbook for a night to review the material (he couldn't remember it all from third grade), and then he walked me through my difficulties and coached me. I ended up pulling a B+ at the end of a semester and avoiding that train wreck. The thing is, nothing he taught me involved raw brainpower. The more I learned the more I realized that the bulk of his intelligence and his performance just came from study and practice, and that the had amassed a large artillery of intellectual and mathematical tools that he had learned and trained to call upon. He showed me some of those tools, but what I really ended up learning was how to go about finding, building, and refining my own set of cognitive tools. I admired R., and I looked up to him, and while I doubt I will ever compete with his genius, I recognize that it's because of a relative lack of my conviction and an excess of his, not some accident of genetics.
It's easy to trick ourselves into thinking that "being smart" is what determines our performance. In so many ways, it's the easiest possible explanation because it demands so little of us and immediately explains away our failings. You are facing this tension without recognizing it. You are blaming your intelligence in the first two paragraphs but you undermine yourself by saying you received good grades you didn't deserve. You recognize your lack of motivation as a factor in your lack of extracurricular activities but not in your SAT scores (fun fact: the variable that correlates most strongly to SAT performance is hours of studying for the SATs). Your very last statement could just as well apply to your entire post:

But none of this has to do with my intelligence; I'm just rambling.

You got A's because you studied or because the classes were easy. You got a B probably because you were so used to understanding things that you didn't know how to deal with something that didn't come so easily. I'm guessing that early on you built the cognitive and intellectual tools to rapidly acquire and process new information, but that you've relied on those tools so much you never really developed a good set of tools for what to do when those failed. This is what happened to me, but I didn't figure it out until after I got crushed by my first semester of college. I need to ask you, has anyone ever taken the time to teach you how to study? And separately, have you learned how to study on your own in the absence of a teacher or curriculum? These are the most valuable tools you can acquire because they are the tools you will use to develop more powerful and more insightful tools. It only snowballs from there until you become like R.
MIT has an almost 97% graduation rate. That means that most of the people who get in, get through. Do you know what separates the 3% that didn't from the rest that do? I do. I've seen it so many times, and it almost happened to me. Very few people get through four years of MIT with such piss-poor performance that they don't graduate. In fact, I can't think of a single one off the top of my head. People fail to graduate from MIT because they come in, encounter problems that are harder than anything they've had to do before, and not knowing how to look for help or how to go about wrestling those problems, burn out. The students that are successful look at that challenge, wrestle with feelings of inadequacy and stupidity, and begin to take steps hiking that mountain, knowing that bruised pride is a small price to pay for getting to see the view from the top. They ask for help, they acknowledge their inadequacies. They don't blame their lack of intelligence, they blame their lack of motivation. I was lucky that I had someone to show me how to look for that motivation, and I'm hoping that I can be that person for you in some small capacity over the Internet. I was able to recover from my freshman year and go on to be very successful in my studies, even serving as a TA for my fellow students. When I was a senior, I would sit down with the freshmen in my dorm and show them the same things that had been shown to me, and I would watch them struggle with the same feelings, and overcome them. By the time I graduated MIT, I had become the person I looked up to when I first got in.
You're so young, way too young to be worried about not being smart enough. Until you're so old you start going senile, you have the opportunity to make yourself "smarter." And I put that in quotes because "smart" is really just a way of saying "has invested so much time and sweat that you make it look effortless." You feel like you are burnt out or that you are on the verge of burning out, but in reality you are on the verge of deciding whether or not you will burn out. It's scary to acknowledge that it's a decision because it puts the onus on you to to do something about it, but it's empowering because it means there is something you can do about it.
So do it.

Source: http://www.reddit.com/r/confession/comments/nxdzz/im_not_as_smart_as_i_thought_i_was/c3d91jl

Very very true.

I'm currently in a fortunate position in my life/career and people always ask me how I do it. There are many factors (luck also being part of it) but one of the main things I harp on is the amount of sacrifices I had to do when I was in university. There were numerous points of time in my university days where I honestly thought I should be dropping out but thanks to my tremendous closeted ego, I always persevered to find a solution to my problems and in that course of perseverance, I also found out a lot about myself, my emotions, my abilities, etc. When I finally completed my course, it dawned on me that university is not about the degree or knowledge itself but about learning to learn, adapt, solve problems and work with people.

I'll be forwarding this post in the future to anyone who asks me about university life. :)