Oh geez..this is how I felt 8 months ago.

It's going to take time. I know you want it now, but be patient. Make a deal with your parents. Tell them you promise to help around the house alot more, and that kind of stuff. Parents like that you're maturing in that type of sense.
They already know you take care of Katie, and your cat but what's one other, right?
Financial problems is what. Right now's a hard time in the economy for money, and maybe your parents couldn't afford to keep another dog right now. You need to understand that.
To get Ozzy, I had to be responsible for 7 straight months. For 7 months, I researched all about agility and the dog I wanted. The hard part was cleeeeaning. I had to do the dishes every night, take out the trash, clean my room, make the bed, clean up Darlin's messes and everything. All for 7 straight months. It was well worth it though! Now I have the doggy of my dreams!
I also have a great deal of respect now. Every time I got in trouble, it would be another strike against my new dog. I had to learn to respect everything and everyone around me, without being rude or troublesm.

Sorry, kiddo but these types of things can't be rushed. Parents are like closed doors..you just have to know how to unlock them, then you have an opening to your dream!