I too agree with finding a *good, supportive* instructor to help you.
There is a book called "Move Closer, Stay Longer' by Stephanie Burns(I think) which addresses this kind of fear, as well as tackling fears that our horses have. She is a psychologist and learning instructor, heavily into the horse scene. I haven't read the book, but have heard many rave about it.
I find that when tackling fears, 'move closer & stay longer' does seem to be effective. I tend to refer to it as 'approach & retreat'. Basically, you push yours or your horses thresholds *a little*, to the point of discomfort(say, fast trot for a few paces), but then 'retreat' back to your comfort zone before you end up over any emotional cliffs, so to speak. Repeat the process until you're confident and comfortable with that level, before doing a little more - fast trotting for longer, going a little faster, whatever.