Hey Pooks, we are definitely soul sisters or something.
I am afraid of speed too. (but I've been riding longer than you...)
On the back of the tandem i am a real pain whenever we are going down a hill
you can hear me yelling
"SLOW DOOOOOOOOOOWWWWWWWWWWNNNNNNNNNNN"

The fact is, the faster you go, the more it's going to hurt when you fall off!
having said that, if you have a computer on your bike, i recommend you take it off for a few weeks, because if you can get up to 11 or 12 mph you're going to enjoy it.
(and if you can't see it on the speedometer, you won't slow yourself down)
for me, now the "Too fast" speed has gone to above 25mph.
so see, there's hope!