Hello Pasty,
I'm experiencing something similar with my husband. I know it's a different situation, but probably there are some similarities.
Are you already seeing a therapist? If not, maybe this would be a good way of bringing up the topic again - tell your mother you would like to see one and why. Hopefully, this could help opening the topic up again.
Also, have you asked her if she has questions, or given her some material to read? I like Sam Killerman's book on gender, for example (it's a free ebook). It's a gender 101 which also talks about trans extensively.
Have you thought about what it is that you want or need from your mother at the moment? Maybe you could tell her - chances are she is uncertain as to how to behave and how to support you.