I had that feeling (F-E-A-R) a couple of weeks ago, after many years and many miles riding, and my share of accident witnessing. The trigger was a very explicit set of pics of a fallen rider literally splitted in two by the armco poles, that here in Spain are mostly sharp-edged steel "I" profiles (many people die here because of that).
As razorshark has said, "use precautionary measure, enforce situational awareness and ride defensively". Standard risk management is the way to go. Reading the books, going to the track, thinking about the techniques, spending some minutes before the ride reviewing the risks, are some of the activities that add fun to the experience, and help me overcome the fear.