First of all, retroactive congratulations are in order here. In my experience, coming out to your parents is the hardest part of the whole experience. I don't know when or if your mother will come around to acceptance, but I can relate my experience with my own mother.
I got the cry-yell treatment too. And the situation snowballed when she told my father. For some perspective, maybe I should mention that once upon a time my parents were evangelical loonies. My father was a pastor even. So when I came out, they kicked me to the curb, and I went from College student to street urchin.
Thankfully, kind souls came out of the woodwork to help me regain my feet. I found love, nearly lost it, and then regained it for good. And six years later I am reconciled with my family and working full time. My father remains distant, but he doesn't actively rebuke me. He's more than willing to talk to me on the phone about whatever crazy ideas I can think up, but he voted for prop 8 out in California. The rest of my immediate family voted against it and made his life temporarily miserable in the aftermath of the election.
The way the progress with my parents went was this:
For the first year and a half, they avoided me. They told me they would open up if I would go to "reparative therapy." I repeatedly refused, and they actively cut lines of communication.
In the second year, our older relatives began to die. Three of four grandparents died within the space of 18 months. My brother called me, crying, after the funerals. I still remember the moment of the call; I was doing laundry with my partner. I burst into tears and she put her arms around me and wiped my face with a cotton nightgown fresh from the dryer.
Toward the middle of the second year, my partner and I moved back east to be with her family. I got a day job and started sending Christmas and birthday gifts again to my brothers and sisters. My parents sent gifts back. Around this time they stopped addressing me as my old name in written correspondence, but they still used it verbally.
In the fourth year, another relative died, and my older sister gave birth to a beautiful baby girl. They asked if we would come for a visit. I got the time off work, and flew out with my partner. It was wonderful beyond words. I held my little niece and she grabbed my fingers for a bit, then fell fast asleep. Everyone was real careful to call me by my proper name and to use proper pronouns, even my father -- though he slipped up a few times and got nasty looks from my siblings. I noticed they use my old name and pronouns in private and my proper name and pronouns in public.
In the fifth and (now) sixth year, things have improved further. My brother told me that my mother finally shifted names and pronouns in private, and that he was proud of her for doing so. My father is stubborn about it, and I don't know if he will ever truly accept me. But with him it's more awkwardness than an actively negative attitude. My grandmother sends Christmas cards to me with my old name on them, but to her credit she doesn't try to convince me of anything. She just tells me that she loves me and God loves me too.
So that's the (abbreviated) story of my parents progress toward acceptance.
In the first 6 months I felt very similar to the way you describe. I felt stuck in that coming out conversation too. I also felt like a failure as a woman because of my voice, because of my height, because of my earnings potential, and mostly because I didn't believe anyone would ever love me. But those feelings weren't rational. The truth is that if I hadn't transitioned, I'd either be dead, or spinning my wheels in the same pre-transition rut and wishing I was dead. I had to transition to move on with my life.
I wish I could tell you parental reconciliation is short and easy, but the truth is that it can take years. I don't think there's any magical argument we can choose that will make them understand. They have to go through a natural grieving process. While we feel newly liberated, they feel they have lost a child. It can take them awhile to realize they have not so much lost a son as gained a daughter.
In my situation, I think moving away actually helped things along. Absence makes the heart grow fond and all that.
So far as therapy, it's probably a good thing to help her grieve. My parents went through grief counseling and that helped them immensely. The pro-T or anti-T stance of the therapist likely doesn't matter much, as therapy typically does not involve the therapist imposing their views on the patient.
