Since you're asking, I'd say, yeah, you should worry. There's a line between confident and cocky; too much of the latter isn't going to help you remember how to do stuff, and won't endear you to your preceptors.
Practical exams mostly measure your ability to follow scripts and algorithms to treat classic prehospital presentations. You won't get brownie points for pointing out minute details or naming obscure conceivable etiologies. Those are things cocky students sometimes do. My advice is (1) practice your stations until they no longer require thought, then (2) repeat #1.
By the way, even if you pass, start a career in EMS and, ten years from now, bask in all of the knowledge you've accumulated, cocky still won't be cool.