Congratulations on that first brave step!
You mom has known you as a girl for 12 years. Don't lose heart. Your mom needs to process the new information.
When I got my diagnoses ADHD and Tourette's syndrome as an adult, my mom said instantly "The doctor is wrong!" But I kept my head cool for 3 months. For 3 long months she stared at me like I was an alien. But then something happened. She realized it was not her fault she had missed my disabilities as a kid, she did not have the knowledge. She started to understand all strange jerks I have.
What I want to say is stay cool, give her time and be confident in who you are. You need to educate yourself to educate her. Tiny hints now and then, not too much info when she is in the denial/processing state.
If you continue to be your true self she will come around. If you can, avoid jumping back and forth with who you are. If you are sure about who you are, your mom will be too eventually.
You can also ask her if she can help you find a gender therapist. A gender therapist is a specialist in the subject, other therapists might not have the knowledge and experience to help you on your journey.
Tony