How to covince your parents to buy you a horse?

I have been riding (barn lesson horses) for 9 years, and I really want to go to this boarding school in Alabama, Montverde Academy, they are totally fine with that, but I asked for a horse and they totally flipped! They yelled at me because they dont have enough money, but my mom is giving me mixed signals....she talks to my trainer about buying tack (obviously, makes me thing about getting a horse) and I ask her about it and again, yells at me for not enough money, she even bought me a whole grooming set with a halter and lead rope and bridle for X-mas. Still... no horse? I'm so confused so I talked to her about it and she said if I pay for half of boarding and all of the food, then I can get a horse.. but every time I talk to her about it she gets mad? I'm so confused. Please help me convince my mom!