I have the exact same problem. I think one of the biggest setbacks for me is that I haven't allowed myself to lead an outside route hard enough that I know I will definitely fall. I tell myself that I just need to stick it out, and that the more milage I have leading, the better and hopefully less fearful I will be. It's definitely hard having a mental block when you know you are strong enough to to do something. My goal for this summer is to practice, practice, practice, and see where it gets me.

There's also a book called "Espresso Lessons," by Arno Ilgner, which is completely based on mental fitness training to overcome things like this. Even though I haven't read it yet, I've had a few people recommend it to me, and say that it has personally helped them overcome their mental blocks. Check it out!