I broke two bones in my back a few years ago when my horse spooked during a lunge lesson. I'm guessing my injuries weren't as bad, since I got back on and finished my ride before going to the hospital. Or I was just dumb about it...who knows. Anyway, I HIGHLY recommend talking to your instructor about this. I'm assuming she (or he) has experience in working with nervous riders and would completely understand why you're nervous every time you get on a horse. Perhaps she'd be willing to find you an easier horse to ride, build your confidence over lower fences, or whatever you two can think of to help rebuild your confidence.

Edit to add: I mentioned the part about getting back on to say that I was lucky/dumb enough to get back on right away and I know it's harder to get back on when you've had to wait to heal afterwards. I hope that didn't come off as criticizing you for not getting back on when you were obviously hurt pretty badly.