Overprotective mother?

i have a mom who (even tho i am 14) doesnt want me to have a girlfrend. i do extremely well in school, and am a pretty good track runner. Plus, i volunteer about 2 hours a week to the community, and i am ASb Vice President. My mom however thinks that having a girlfrend will distract me from my goals. And while i do want to continue to excel in all that i do, last year i had a g/f behind her back and my grades were better than ever (all A+'s)! anyway, i want to explain to her that i am mature, and responsible enough to have a girlfriend without losing my focus on academics and athletics. Also, i am mature enough to no that things like are a big no, no. how do i tell her.