[gloves123]

it's often very difficult for the older generation (parents, mostly) to understand and accept gender identity. So it's important to keep that in mind when having discussions like that with your mum, but it's also inappropriate for her to be acting the way she is.
it's tough for parents to get used to their children changing their names and or gender identity, but hopefully she'll get used to it soon.
Explain to her that that is the name that makes you most comfortable and that it bothers you when she invalidates that.
I hope she understands sooner rather than later!