There is only one thing to do, and that is, to get help from a psychiatrist or competent therapist. You should do that right away. You must come to understand why this is coming up now, what it means, and what you can do about it. You can not do this alone. This is very serious. YOu should talk to your parents about a therapist or go to the counseling department of student health at your school. In general, you would be better off with a private therapist because those at school may not be able to see you enough, or long enough, because of scarce resources. That depends entirely on you school.
One unprotected sexual experience does not mean a death sentence. There may be no physical consequences at all. See your doctor if you are still worried about this.
But, by all means, get therapy, and get it NOW.