I know that it sounds hurtful people close to us seeing transition as them losing a person. Especially as in reality they aren't - we are the same person regardless of how we present - and it can feel as if they don't really understand us.
But looking at it from your mum's point of view, she sees herself as losing a son, and gaining a daughter. Remember that she's seen you as a male all your life, even if you haven't, and thats gonna be difficult for her to see differently, especially in the short-term.
Not sure what advice i can give, other than give her time and she might begin to see things differently. Also maybe her just seeing that you are still you, even as you transition, will be more effective than telling her that your still the same person.
But that takes time. And i know thats frustrating. Hugs.