I'm too fearless on descents on the bici
, but I can totally relate to your post, because a couple of years ago I completely lost my confidence on low-speed maneuvers on the motorcycle, and as you say, it's really taken all the fun out of riding. 
I'm not sure of the solution, either. I know intellectually what's going on, I'm panicking, freezing up, looking at the ground and tightening up my muscles - when what I need to do is be loose and look up - but it's the panicking part that's hard to conquer. "Practice" isn't necessarily the solution for me either, since it just tends to reinforce frustration and fear.
I guess the one difference is that in low-speed maneuvering, if you panic, freeze and look at the ground, you WILL fall... whereas descending, you can control your speed with your brakes to a level you're comfortable with. I guess that's what I'd try in your position - work on the self-talk, pick your eyes up, look where you want to go, forearms loose, shoulders loose, weight in your quads rather than hard on the saddle; on the curves, inside foot up, outside foot weighted, still remembering to look where you want to go; and just every time down your nemesis hill, go one mile an hour faster.
Definitely DON'T unclip... right there you're losing a lot of control over your bike. And remember that you DON'T have to hit 50 mph if you don't want to! 
ETA: I will say that riding a moto is the main reason I'm so comfortable descending on the bici - because it's all about looking where you want to go and being comfortable with the speed. Short version is that I burned out on the bici, rode a moto for 12 years, then came back to bicycling and my very first day back on a road bike I was descending gleefully at speeds that used to terrify me when I was racing. I wonder if you're being sloppy with your eyes when you drive your car? Maybe that would be an exercise you could do without panic... practicing your awareness skills when you drive the car? When you know how to look where you need to look when you're going 70 mph (or faster
), 50 doesn't seem so scary even if you're not wearing leather.
Last edited by OakLeaf; 12-01-2009 at 04:08 AM.
Speed comes from what you put behind you. - Judi Ketteler