The reason she is going faster down hill is because she is not fully balanced yet. Just like a person going down a hill, if you lose your balance you wind up going faster and stumbling. What I would do is hold her head up high and lean back and keep your hands up and have a steady contact with her mouth, if you feel her speed up, sit back deeper in the saddle and lift her head up more and keep a good grip with your legs the whole time. She is a young horse so she doesn't know how to balance herself that well yet. She'll get it.