I totally agree with you that you are perfectly entitled to have your hair however you want it. But you can't change your mum's way of thinking now, and apart from a wig you are saying that only way to make her happy is to cut it. If you are willing to make that change for her sake, then why not try to turn it around and make it into your decision - have a look at some hairstyles, and spend some time choosing a short style you like. If you can find the right one, maybe it will even suit you better than long hair when you've got used to it. Try to take some control back so that you are not just doing it because she has put you into such a difficult position.