I have a few students this year (and every year) whose main impediment to learning happily and calmly is the pressure they feel from their parents. One had been getting low C's and D's for a while, but worked really hard and got a respectable B- on a tough test. But he was unhappy—his parents have conditioned him to feel like all hard work will always earn A's, and anything less represents some sort of failure. I have another student (I'll call her Andrea) who is a perfectly fine math student. She earned a B- for the term, and will probably stay in the B or B- range. Getting an A in this class at my school probably puts a student in the top 10-15% of all the math students in our entire major metropolitan area. Andrea is not quite of that caliber, and she easily accepts that. But she's no slouch, either, and she works pretty hard, and she does fine. But she came to me worried about her grade, because she thought that if she got some form of C for the term, her parents would "literally kill [her]". "Literally" is obviously an exaggeration here—I don't think they'll even hit her or anything like that. But I do know they put an extraordinary amount of pressure on her. The only reason, I suspect, that they were okay with her second place finish in the state competition in her sport was that she lost the in final round to her older sister.
So. An open letter:
You are a good math student. In fact, at some point, you're going to have to face the fact that you're probably even a better math student than you think you are. I know you've been conditioned to believe that getting an A is the only acceptable evidence of being a good student, but I've been doing this teaching thing for a long time (about as long as you've been alive), and I want you know that getting an A isn't everything. You work hard, pay attention in class, and you're intellectually curious. If you weren't worried about your grade, I suspect you might actually enjoy math more than you let on. I would rather have a room full of students like you than a room full of A students who are apathetic and jaded.
I know that with some encouragement, you can probably come to accept your grades and even be as satisfied with them as I am. But I also know that your parents are a different story. They control most aspects of your life, and you have to make them happy. I will be glad to tell them everything I'm telling you, but from what you've said, that might not convince them (at least right now) to cut you some slack. Objectively, it seems silly for anyone to think that you're going to be one of the top students in every one of your classes for your entire life. That's just not possible for normal people like you and me. Your parents will understand that eventually.
But yes, that doesn't solve your problem, does it? You have to deal with them now. And you might have to deal with them until you leave home for college, or maybe even longer than that. So my best advice is this: you have to see your parents' pressure on you for what it is. They love you. They want the best for you. They have some very specific ideas about what 'success' means, and those ideas involve doing everything right all the time.
They're human beings, and so they're flawed. And you love them, so you have to accept those flaws. They might make your life miserable in a lot of ways, but it will not be forever. Believe your teachers. Believe that voice inside yourself that tells you that you really, really have done your best, even if you didn't get an A. Find ways to gently bring your parents to understand that you can't be the best at everything all the time. No one can. Don't yell at them about it—that will just make them dig in to their beliefs more strongly. Playing the angry teenager just reinforces their idea that you're not mature enough to understand their position that you could be working harder and getting A's. Your teachers will back you up on this if you really do continue to make a genuine, sincere effort.
It's hard for any teenager to see his or her parents as just regular people. And you won't be able to do that completely for a long time. But sometimes, regular people are just...wrong. And that's what your parents are now. Their pressure comes from their love for you, but that doesn't make it right. Keep making good decisions in your life like you are now. This will help them trust you more. You will eventually earn the credibility to say "Mom, Dad, sorry, but...I really did the best I could, and this is how it turned out. And if it means I don't get into MIT, then, well...that's okay with me. I'm satisfied with my effort and with the result, and I hope you can be, too."
Your teacher, who has seen more students in your position than your parents have.