Hello! It's good to hear from you.
You're feelings are very valid and normal. You may like this psychiatrist, but you have not established a bond of trust yet, or if you have, it's new. Plus, repeating this story not only is difficult, her reaction is up in the air. You mention telling your friends about your attack because they would not hurt you. But you do not know the same about your new doctor. The fear of getting hurt over something so painful is very real and very valid and it's natural to protect yourself.
It is not odd at all that you feel more comfortable letting your therapist tell her. That way, your therapist can gage the reaction. Plus, you may feel that your therapist would protect you from any negative reaction (which I doubt your doctor would have), which is a comforting feeling right now.
There could be a chance she would refuse to hear your story from your therapist, but that is highly unlikely. You are already being treated by your therapist so there is no motivation on your part to avoid dealing with how you feel. And refusing to allow your therapist to tell her would be based on personal feelings, which is not a good enough reason. Your therapist would pick up on that and do something to protect you, if that was the case.
She may bring your attack details up with you just to try to help. From your description of her, it does not sound like she is anything but kind and caring. So far, she has put your feelings first and tried to make your experience good with her. I can't think of any reason she would try to hurt you or try to force you into a situation you are not comfortable with.
You may want to keep your appointment and see what happens. It is ok to tell her, if she brings it up, that you feel uncomfortable talking about it right then. She should understand. You control how this is discussed and handled. You are the client and this is about you. If someone tries to do something that bothers you, it is perfectly fine to say so. But from what you have said about your doctor, I think she will respect you and your feelings and not hurt you in any way.