I show with the IHSA, so I have to get on a horse I've never met before each time I show. Yes, it can be really disconcerting! I think it's good to get shaken up a little, though. You can get into too much of a groove when you're always on the same horse, and that can lead to bad habits - especially if your horse is really well mannered. Take it as a learning experience, and hopefully your girl will be back on her feet in no time!