Here's my 2 cents, and that's all it's worth since I don't know you and your mother personally and this is an intensely private thing. But still, you asked

I'd tell her face to face, again, but I would do it as simply as possible and I would provide her with an immediate course of action so she has something to focus on. Like, "Mom, I'm transgender. I think I need to see a therapist. Can you help me with that?" Or something along those lines. She's probably going to pelter you with questions, concerns, etc., but maybe if she has something to do to focus on she will focus on that instead. Also, I would keep it as simple as possible. I would think the more details you give her, the more ammunition she has to fire questions and doubts back at you.
Again, just my 2 cents. Good luck sweety