Hugs, Kit. I started trusting Dr. T pretty quickly--I think it was our third session (seeing him weekly) where I was very open with him about the stuff with my former marriage counselor. I kind of freaked out after that session, as I felt I'd shared too much, worried he wouldn't want to keep working with me, that he'd be freaked out, etc. Sent him a frantic email regarding some of that. But it ultimately worked out.
Sometimes, you just have a feeling that you can trust someone. Like you said, it can also be a case that you're at a point in your life and therapy where you can trust more easily. I think for me it helped that I'd shared much of the stuff already with ex-T and also ex-MC. So I wasn't starting from scratch and telling someone about something for the first time.
This would be a good topic to bring up with her. About your fear that trusting her and being open will lead to something bad. Do you maybe fear rejection? I know that was a big part of it for me. But I think talking about it is the way forward.