It is wonderful that you have come to the forum to chat and to get some support, it is a wonderful place and I am sure that others will come along too to lend you an ear and a place to feel comfort.
I too have a 13 year old daughter so I know that is not the easiest of times, let alone when she is crippled with anxiety. I am so sorry that you are going through this, I dont know how you feel but we are here for you in this really challenging time.
One thing that I might suggest to you is for some communication with her when things, not matter how small or insignificant you may see them as, change in her life. Whether that be from things in the house, like the heater arriving or even a different meal that you have not cooked before. Just a simple conversation with her so that she is prepared and so that it does not catch her by surprise and trigger her.
Also there are the wonderful people at Kids Helpline that she could engage with until her appointment, if she does not feel comfortable to chat to them they even have a web chat service. I believe that at this time they are really overwhelmed with people visiting so there may be a wait time, but it is a service that is very worth the wait. The phone number if she feels like a chat is 1800 55 1800 and I have put the link for the web support here too:
She may not understand or even know why these things trigger her or what it is that she is "afraid" of, if that is even what she is feeling, so maybe asking her is just making it worse for her as she does not know, or cannot express herself. Sometimes just being there with her, sitting in silence with her just so she knows you are there, you love her and you want to help her, in whatever way that looks like for her.
In saying that you could try and ask her, I am sure you have done this already so apologies if I come off as "sucking eggs" here, it is not my intention, when she is not feeling overwhelmed what you can do for her in those times, what she does need from you as you know what does not work, maybe she can help you to help her.
Maybe even a book that she can write in, journalling is a wonderful way to get stuff out, she may choose to share this with you, she may not but it could come in use too when time to see a therapist to help her explain what she is feeling and thinking, just an idea.
We are here for you to support you through this and I hope to chat to you some more.