Help with my anxiety about grades

I'm 15 and in grade 9. Ever since grade 4 I made it my mission to get the best marks possible. I'm an honor role student but I have a huge anxiety when I get a "bad" grade. If it's under 80% it feels like my life is over and im a total loser who shouldnt even exist. I start to freak out, cry, rub my feet raw and hyperventilate. I'm not trying to seem stuck up or anything it's just something that I do. I'm afraid that if I keep it up I could do something and injure myself without meaning to. I just get out of controll. Please if you have any suggestions it would really help.