I do i convince my parents to get a horse?

Okay, So I am 13 years old and i have been riding for 8-9 years. I have 2 pony's one is 10 hands and the other is a miniature and is 29 inches tall. I really want to get another horse that i can actually ride! I use to have this horse named skittles.. I loved her ALOT but she was still green and i was really little for her even though i was like 10-11 i was like 4'6 and she was 15 hands tall so i looked small. And also she couldn't jump that is another reason why i sold her. So my dad said that he would get me a horse and a reasonably priced horse trailer. But he doesn't want to pay the cost of like boarding, shoeing ,and vet. (my parents are divorce). I have my 2 ponies with me and my mom. But i really want to get this horse and board it at a stable where my friend rides. I would be willing to let them use the horse as a school pony and i could help do barn chores to reduce the cost of boarding. But i don't know how to convince my mom to get a horse. Also the place where i would board you can donate horses there.. so i am thinking of donating my ponies there because i dont know what to do with them when i am in college. I no it is weird for me to think about college but i am. So please help me on what you think i should do!