How can i get my parents to take me serious about getting a horse?

ive been riding for 6 years now going on 7. im starting to show on of my trainer's horses for her and everytime i bring up the subject to my parents about me chipping in or even just mentioning-oh look how pretty that horse is-my mom flips. my dad is ok with it and would love for me to have one but my mom is always-no its too expensive and we cant do this and that and blah blah blah-i know that we have the money and i know that i am responsible enough and it brings me to tears whenever i think about her not taking it seriously....how do i tell her my feelings with out getting mad? and how do i get her to just listen to me?