I have to put my hand in for the fear vote as well. So much of the stuff I know I can do physically, but it's the mentality that I struggle with. I try to visualize myself doing a monkey stand onto a picnic table say, and all I can visualize are my feet clipping the bench and the many places my face will probably land. However, when I actually do it, I could. It's just a matter of jumping that first time. Mind you, I have smashed my knees multiple times, but the only actual wipe out was when I was doing a jump to a bar that was diagonal and the side of my knee went full speed into a metal pole on the side. Ironically, that's not what I kept visualizing as the wipeout potential. But yes, fear is by far the biggest one for me.