How to convince my mom to flat iron my hair?

okay so i have the thickest curliest hair and it was been in braids all my life and like the beginning of the year she let me flat iron it and so i had it in for a long time and then i got it went and no she wont let me flat iron it Again and she says that is damages my hair which it does but not to my hair because i use bio silk and she just does not believe me and i really want to flat iron my hair like i can take care of it but she wont let me do it how do i let her no that iv only flat ironed my hair once and this well be the Second time that its not going to to kill me what do i say to her and how do i say it!!>???? PLEASE HELP!