I also have this problem. I am extremely nervous and can't find the proper things to say to anyone superior. I put it down to my strict upbringing. ie:: speak when you're spoken to, do as you're told by doctors, teachers or else. Everyones different though, thats only my take on myself. I now tell myself, they're not superior as such, they have better positions in society, but have found most of these people down to earth, but a few.