Hmm..I got lucky with my dad. After leasing my second pony for a year or so, I started slyly showing him for sale ads, leaving them up on the computer, etc. Luckily he liked horses and spoiled me, so I finally got him to actively go on the hunt with me =P Mom wasn't too happy, but what's new ;) Granted, she was cheap and older, but perfect for a couple years 'til I was ready to move up. She also taught me a lot and I wouldn't have advanced nearly as much if I'd had a very old or very young horse that I couldn't learn on. If you have the resources for another horse and you're sure you want her, just keep trying.
Lol sorry, terrible advice. I didn't already have horses when I started trying to convince my parents to let me have one, so it was a little different.