Like Beth Andrea said, it's your hair, and your mom should respect that and not force you to have your hair cut short.
Anyway, since it's already been cut, just remember that you don't have to have long hair to be feminine. I'm AFAB and first got a pixie cut back when I was a 10-year-old kid who just accepted "I'm a girl because I have a girl's body" and didn't really question it or know there was anything besides male and female. My mom, aunt, and both my biological grandmothers all have short hair as well. I'd suggest perhaps looking up pictures of women with short hair if you feel you need a reminder that short hair can be feminine, too.
Also, as awilliams1701 said, guys can have long hair, so perhaps you should remind your mom of that. Funnily enough, my brother actually has the longest hair (almost shoulder-length) in our immediate family, and my boyfriend's hair is far longer (several inches past his shoulders). Really, she shouldn't make you have your hair short because guys are "supposed" to have long hair or whatever, since plenty of guys DO have long hair.