"you're too pretty to be a boy" would be like someone said "you're too butch to be a girl", I know how that could feel. If you feel like you want to have short hair, then do it, people will accept it sooner rather than later, and if they can't, too bad for them.

It's great you found cosplay to express yourself, and I might be wrong here, but I think girls have more freedom to express themselves than guys. i'm no expert, but I suggest try be as much as yourself as you can.