@outlander I've been thinking about your question here since yesterday because I wanted to make sure I answered you in an ok way. I think it's ok to say to the psych something like ... "Something terrible happened to me when I was years old and I'm not ready to talk about the details just yet because it's too traumatic for me and I have no supports away from here. But this is how it's affecting me ....." and tell them about the tremors, flashbacks, nightmares, anxiety, depression - the symptoms and functional effect of what happened that are in the present. Hopefully your psych will be sympathetic, empathic and gentle once they understand how hard it is for you. And not pressure you to go where you are uncomfortable at this stage. The most important thing at the start of sessions (in my opinion) is developing some trust (that goes both ways) and establishing any boundaries you need. A good therapist will help you to feel safe with them.
I've seen many therapists over the years and some of them have done exactly this with me. Ask me any questions you want to and I'll try to help and support you. I urge you to go to the appointment and will be here for you as much as I can.