I've gone into therapy with a new therapist (in fact it was my current one, 10 years ago) that we weren't talking about my past until I was ready. He was fine with that and over time I became more comfortable and now we've discussed nearly every detail of every traumatic memory I have and it was safe and incredibly helpful. Tell them upfront that you need to take some things at your own pace and if they aren't ok with that you need to find someone else.