downhill for roadies.....
Going downhill reminds me of skiing. It's a control thing, I think. I have a friend who crashed hard while skiing - and unfortunately that transferred to the bike. She has the ability to ride downhill - but the fear consumes her. I've watched her walk her bike downhill before - and it is something that only she alone can overcome. Caligurl had a great suggestion for when the fear is too much - conquer it in increments.
For me, like skiing, downhills are the reward for all the climbing, headwinds, and stinging rain. I enjoy each nanosecond of it. Like standing at the top of a double black diamond - where anything and everything will be thrown at you and your legs, you just have to do it. Sure you may fall at first - but practice on the smaller hills. Work your way up - slowly and at your comfort level. Just make sure you have a grin on your face afterwards - or back it off a little. Whatever you do - don't get in over your head. Otherwise your fear may cause a minor error that could turn into a major issue.
It's the best when you're running 53-11 and can't keep a high enough cadence to push any more. I make sure my equipment is in tip top shape - my brakes work, my hub is running smoothly, my wheels are perfectly trued and my helmet is tightly strapped beneath my chin. That way the fear of equipment failure is pacified while going 40-50 mph. I am one of the crazy people you see screaming down the hill - I know, I should have more sense. But if there were downhill road races - I would be an Olympian.