If I were you, I'd lay off the jumps for a while if possible. Do simple dressage exercises and develop a bit of a routine - so that there won't be anything new in the lesson that would scare her. That way, her confidence builds up because she knows what you want from her.
Then again, if she isn't a problem at all with usual exercises, then maybe - jump only one jump with her, preferably small one, and don't introduce any other jumps to her for a while. Yes, it will frighten her but it's the same with people - each time you go through something scary it becomes less and less so with time.
Also, I'd suggest jumping fences that you're comfortable with :) because the rider transmits her fear to her horse. It might not actually be your horses fault at all - you might be getting nervous about the fence/falling off yourself - and becoming tense. That to the horse means 'uh, oh, my rider is tense, there must be something scary coming!' and that could be why. then again, I haven't seen you riding so I might be wrong.
Just take it slow and try to be relaxed :) good luck