Make sure to help her understand that the effort she's put into ballet won't have been wasted if she doesn't make a career of it. I'm a successful banker, but I certainly don't regard my years in ballet as wasted. I'm still reaping the physical benefits (at 44), and I certainly think I got a lot more out of all those hours at the ballet studio than I would have gotten hanging out at the mall or practicing/playing on some school sports team.
Oh, she hears me tell her that even if dance doesn't turn out for her, I will not think it was a waste because I would rather have her busy with dance than at a mall.
I want to plant that seed now so that she won't feel bad if that is how it all turns out. She loves it so much.