I know how you feel...I took me 2 YEARS before we got Molly.

I did odd jobs around the house (cleaning, making beds, doing dishes, sweeping, etc..), took care of all the pets, AND had to get excellent grades in school. When I did all that, my parents thought I was ready. Getting Daisy was a little easier, because we thought Molly needed a friend to play with while we were gone. This possible new addition took FOREVER to convince; but we finnaly came up with a plan (get good grades in school, I'll get it.)

It will take some time. But it's well worth waiting for. My advice to you is just lay off the subject for a few months, get good grades in school and then ask your parents again. And if you never get that other dog, be happy with what you have!