I think its worth pointing out that she has had a long period of time to come to terms with who she feels she is and how she identifies herself.
For other people they need time to adjust to her way of thinking. You can't just click your fingers and change how you view someone and address them. Even the most liberal minded and accepting. We do have socially constructed preconceptions we have to lose unfortunately. We do have hopes and dreams for our children. We do have fears and concerns that we don't want to accept as a reality. Because we are human.
With the best will in the world, you need time to adjust, and she has to suck it up and accept that it takes time to do that, and that different people need different time scales to do that.
You can't just operate to her time table. Her expecting you to, is indicative of immaturity and a lack of awareness of issues.
Just as you are unaware of issues too.
If she is serious, she needs to give you plenty of free passes to putting your foot in it, because you will. Lots.
Personally at this point, I think you questioning whether this is a phase or a long term thing is a reasonable and indeed wise thing to do, even if she doesn't see it that way.
Your job, as a parent, is to be her guide and to try and advise her as best you can. Sometimes that might mean that she doesn't like what you have to say. It doesn't mean you are rejecting her or not accepting her. Merely trying to make sure in your own head that she is doing this for all the right reasons and you've done the best by her. You have to be tactful in the way you do this, but there the principle of you needing reassurance too is valid and important.
Being supportive is not just about nodding your head and saying 'I'm fine with that', especially if deep down you aren't. Being supportive is, saying 'I'm struggling with this. I love you. I need to know you are doing the right thing too, even if you are sure of it. I might need to be told this a lot but this is because I love you. I will get there in the end and I will listen to you. This is going to be hard for both of us, but we do this journey together'.
One of the main differences between being gay and being transgender is that nothing really changes with being gay, but with the other, you are talking about hormones and surgery which lead to permanent physical and visible changes. The ability to 'change your mind' is the key difference. A significant number of trans people do have regrets sadly as expectations and reality don't match and this is an important contributing factor to suicides.
I'm sorry if none of this is pc and offends. Its tough shit. I do think it is a process that has to be dealt with and there is more than one side to that process and is about different groups of individuals.
I think its worse not to acknowledge that at some point the chances are that everyone on all sides is going to upset someone else either deliberate out of fear (though still through love) or out of ignorance or pure mistake. What counts is stressing the point that acting out of love can take many forms and that's what really counts and to accommodate for that and keep remembering that at all times.