Sailor has a point..maybe you can sort of ease into it.
If you are on T, it won't be long before you can't hide transition evidence, and will have to come out anyway.
If you can, gather a few family member as allies to back you up. Remember, it will likely be a shock to her and she will have to grieve losing a daughter, so it might take some adjustment time.
Maybe if you are matter of fact about coming out to her, she'll accept it. But if she starts saying hurtful things, you could say something neutral like "I'm sorry you feel that way, but of course I must true to myself to be happy," or some such thing.
If she becomes even more verbally abusive you can say something like "OK, it was great seeing you, but I have to get to an appointment now. Bye! I love you!" and just leave.
Most women throw fits to get their way, or as a means of controlling people, so if she sees that your gender presentation is not negotiable, and that you aren't going to let anything she says change things, she might resign herself more quickly.