how do you know you won't be a good employee? Have to give yourself a try. Let's be honest that's friggen hard if you lack in confidence. I would say just be yourself, I was always taught that and I guess it worked for me but I don't know how confident you may be or how you come across (hope I'm wording that right without sounding condescending, sorry)
But if not as others have stated just fake it, most people bullsh*t there way through interviews a little bit, be humble and always try to seem enthusiastic. (Admittedly that's hard as well, haha)
anyway hope it all goes well :)