Hang in there Frogslegs! The first step is always the scariest, but it's the nurse's job to help you feel more at ease. Be open (even with your fear), honest, and willing to try new things, and you should get through this appointment. Don't hide anything, even the ugly stuff, because if you are bipolar, then you have a better chance of getting the proper diagnosis.
Don't despair if it takes time to get your answers. They hardly ever get announced on the first visit.
You'll be okay, just keep telling yourself that this is not going to be a disaster. (Or what ever phrase you need to keep you from panicking.)
Come back and let us know how it went!