how do i become a smart person again?

i have gotten all a's since 5th grade and now im in freshemen year with all a's and one c. in math. im in honors math and have been soosososo good at it. i feel like quitting piano got rid of my brain. i feel like mush all the time, and i just don't feel myself. i feel like i won't do good on the midterm this week. i've studied so much but i still don't get how to set up the equations for linear and quadratic story problems, and i just feel plain stupid. yes, i will admit that my teacher sucks, but my dad's sitting there at home helping me like crazy. he's amazing and i just feel like i don't take enough advantage of his smarts to learn as much as i can. i just get so frustrated that i don't get math anymore and i need an a! i don't know what im gonna do. my midterm's in two days and i've been studying for the entire day for two days but i feel like i haven't mastered everything yet. i have a chance of getting that c up to an a only if i ace my midterm completely. otherwise i don't have a chance and im just so worried.