Convincing the folks to buy me a horse...help?

I've been riding for 9 - 10 years now. Showing for 4 years. Yet, i still don't have my own horse. I cant jump higher than 3 feet because i constantly switch horses. My parents dont want to buy me a horse, despite my profound knowledge and maturity of my age. I get all A's and B's. I'm mature and respectful. I just need some help showing/convincing them to let me buy a horse. Thanks.