I think I know what you are talking about. I had an uncle who use to come to our house so my mom (RN) could give him an insulin shot, when we were kids. He was the toughest scrapper you ever saw, until presented with a shot needle or a little drop of blood. He'd go pale and feint!
Like any phobia, I suspect confronting your fears head on would help but a call to a shrink would be my first step. My uncle REALLY would collapse, uncontrollably.