I would still voice your concerns to her, she has to know. A good coach will always listen and not push you do to do something your not comfortable with. If she still wants you to ride a different horse, before you leave ask, so who do you think I will ride next week? That way, you can plan a bit, and if she says a horse you are really uncomfortable with, tell her you really don't feel comfortable on him and you would like to ride someone else.
Good luck! We've all had drops in confidence (well, I have at least) and I find the best way to approach it is to keep telling myself, I am a good rider, and I can do it. Say it enough times, and you will notice a difference!