Why do i have a lisp?!?!?

i am a 15 year old girl that's a sopre in high school. i have a lisp and i don't know how to make it stop. people bring it up to me all the time, and my friends try to help me say words right. when ever there's a S in anything i cant really say it, and i know its annoying to other people. i am sick of it. i hate when people try and help me and i hate that people bring it up to me that i have one. iv never had braces and i don't wear a retainer so why do i have one?!