Are you still in Highschool? I'm asking this because you said you don't like your teacher.
^ I just want to know this so I can tell you the wonderfuns of University
You're not the only one that has felt or feels like this. There's always going to be someone better, in one way or another, but you're also going to be better than someone else. I went to a small highschool and I graduated top of my class, but when I entered University, I became 'nobody'. There are people who have perfect or near perfect GPA that balance school, work and even volunteering. It is frustrating seeing my own grades not being as high as I'd like them, but it is a lot of hard work to get where they are. I have friends that get better and worse marks than I in some classes but better in others. Don't fret about it.
Anyway, don't choose something that you're not that interested in to be your CAREER. You're going to be paying money to be educated in that field, you might as well make it one that you're going to enjoy doing for the rest of your life. Saving a couple thousand isn't worth being miserable the rest of your life (I don't know what tuition is like, but it's not that much different where I'm from). Also, don't choose too many courses at once. As nice as it is to experiment, or get done faster, or whatever, your marks will suffer. So what if other people can do it? Know your own limitations.
"Even a broken clock is right twice a day."