I am dealing with this now as well, I use to be brave, I think. Rode any horse, retrained it didn't bother me a bit. Having great balance with or without a saddle I rarely if ever fell. But the last few years of riding lessons I started to get hurt a lot, bolting, rearing, bucking lead to many trips to the hospital for x rays and stuff. Nothing serious but after two or three years of that every lesson it took its toll on my confidence.
I just got my new mare, I bought because she was calm and could care less, but responsive at the same time. I got frustrated because my balance was suffering, so the other day I asked for a leg up, I forced past my fears of being hurt again and have been riding bare back every day of the this week. I shake every time I get up there, but Spice doesn't bat an eye lash and I am now trotting around confidently. Mind you it will take awhile before I am 100% again, but I will always be more cautious.
I like the quote the Joe4d put by John Wayne. "Courage is being scared to death but saddling up any way"