She's your parent, not the almighty dictator of your life. If you like it, that's the only thing that matters. You haven't done anything wrong, you haven't hurt anyone, and you haven't lied to her. Trust me, I've had all the hair lengths: waist-long, down to my shoulder blades, a bob, a crew cut, and Ivy League styles. My mum didn't like it when it went above a bob (screamed at me saying how I was embarassing her, looking like a boy). And guess what? The world didn't end and I'm growing it out again because I felt like it. Anyway, even if your hairstyle was terrible, you have to make your own mistakes in order to learn.