I have the same problem. I am new to cycling and I get very nervous going downhill. I'm frantically scanning the ground and convinced I'm going to hit a pebble and go flying towards the ground. 
I made myself go up and down one hill over and over again the other day. It seemed to help a LITTLE, as I didn't brake the whole time after a few . . . but I am still a worry-wart about it.
Any tips on how to overcome this fear?
"i believe in pink. i believe that laughing is the best calorie burner. i believe in kissing. i believe in being strong when everything seems to be going wrong. i believe that happy girls are the prettiest girls. i believe that tomorrow is another day and i believe in miracles." --audrey hepburn