I also have a vertebrae (sp) that was broke many years ago (almost 25 years now), it is broke in 2 places, and never healed right. I ride 2 ways only: I have a custom made western saddle that my Dad made, it does not have a padded seat, but the way he put the rigging in allows me to keep my feet ahead some, and I ride with all of my weight on the ball of my foot. The only other way I ride is bareback.

I lit pretty much the same as you did. I ended up landing 5 feet in the air against a slab fence, then came down on my elbow and hip, which cranked my back and broke the vertebrae, to this day there is still pieces floating around lol. To add insult to injury, she came at me with her front feet after, so I ended up rolling under the fence, which did not help my back any. It did not hurt for about 24 hours, so I didn't go see a dr. until it was too late. The dr. said that if I hadn't got back on and rode with my back already hurt, it probably would have been ok, but anger is a wonderful pain killer.

One thing you might want to try, is something like this, it might allow you to ride longer at a time, I am not sure if it is allowed in competitions though.
http://impactgel.com/seatsaver.html

They have some that are for motorcycles as well, so that give s a few size choices.

http://standardbredhorses.yuku.com