One thing you can do is to open and close your fists. I know when I'm panicky, scared, over anxious, I can feel this too. Doctors say that this action of making fists, opening and closing your hands can help. It pumps your blood and calms the nervous system. Deep breathing is also helpful. The key is to breath in for a count and hold it and then breath out for a count (two?) and hold it. And to do it a few times.
My son is a bit younger than you, in 8th grade. He very much cares about his academics. Which I, as a mom, love. Except that he is anxious about it. He's a perfectionist. We are working to focus more on process rather than the perfect end result. This helps him mentally shift and get past the overwhelming fear of not doing well with the grade he receives. Perfectionism is a trap! Important to have a growth mindset too verses that you are only as smart as you were yesterday. Growth mindset is helpful to my kids and their perspective on school. I can explain it to you if you are unfamiliar.
Do you have a school counselor you can talk to or what about your mom? I know I live for the moments my son 'shares' with me and lets me know what is going on. She may be much more helpful than you think!
First, as in a prior discussion on this forum, you really might not and probably aren't having any heart acceleration. It might feel like that, but that doesn't mean it's happening. The same is true for cortisol -- this usually happens during a panic attack, not just because you feel anxious. And you do have clinical anxiety if you're chronically anxious, so I'm not sure who told you that. Look, nobody is perfect -- so you're not going to be perfect no matter how hard you try. On the other hand, having drive isn't a terrible thing. You're also young enough to have plenty of time to play and exercise even while taking a college class -- I always found time to exercise even when I was in law school. All work and no play, well, you know. What you need is a good psychologist so you can both get to the bottom of why you feel so insecure and also techniques to not be so anxious so that, over time, you might learn to not think this way anymore. It won't happen quickly, but you're young and young people are very adaptable. And get out and move -- burn off some of that angst. It will only make you more efficient when you do your schoolwork.