She won't want to be rid of you just because you did badly in exams.
maybe you just have low self esteem! this supposedly amazing girl chose you so you must have something going for you even if you can't see it. Perhaps you could ask her what she sees in you?
And any decent girlfriend wouldn't give a flying **** what you get in your exams!
Moaning and whining is probably going to annoy her more than getting bad grades.
If she wants to break up with you, then she will. If she was upset with you, trust me, you'd know. So what actually is the problem, then? I think you're just feeling insecure because you obviously care for her and have begun to feel inferior, and therefore 'unworthy'. But honestly, it doesn't seem there is a problem; so don't create one. Don't let your insecurities drive you to self-sabotage.