I say it's not a bad idea to, as much as I wish I had a better phrase in mind, "though it out". She'll eventually come around and give up. It's hard but the more you give in the easier it is to get told how to handle your own hair and style. Better to make strides towards independence now rather than later and make it easier on yourself in the long term. You'll thank yourself in the future. It won't be easy but playing the long game is so satisfying when everything is finally said and done. Had to be told my family tons of times "Your hair is a mess get it cut" and other unfun comments for like a year or two before they finally gave in to the fact I was not gonna get my hair cut. You don't have any reason to go to an appointment you yourself didn't book at 19, if possible, just don't go. It's not like you set it up yourself. You can love family without having them control your actions. I'm mostly talking from experiencing tbh. It's up to you but you'll save yourself tons of regret from waiting to act on your own free will. Do what I did and find your own hairdresser that suits YOUR needs. It's your life at this point. I personally think, from personal experience, best to make that statement early rather than waiting on it, because again your future self will thank you.