What does she want from you? For you to keep seeing her and paying her for every session, of course.

From your previous postings on the subject it seems she is pretty clueless about what you want or need, and she obviously doesn't share your sense of urgency. I'd advise you to start the session by reminding her that she has said you're ready for the transition, and politely ask her to write you the T letter there & then. She might have simply forgotten that she'd agreed to it because she sees so many patients - so give her a nudge.
Explain to her that you have certain deadlines by which you want to achieve certain goals. Explain how the delay in starting T is having a negative impact on your life. Write down the timetable to which you'd like to work and take it in with you. She's not going to take hints from you: she needs you to be specific and clear about what you want. At all times, be friendly and polite, but firm and confident. If your mother backs up what you're saying, it might finally sink in with the therapist.
And if she messes you around again, ask her why she wants you to delay further - then change therapists if you don't like the answer.