I had this same problem when I first started therapy! I found that keeping a journal between sessions of what I wanted to tell her/ things I wanted to talk about was a really helpful tool. I'd either let her read it, or I'd read it to her. That gave her a starting point without me having to struggle to gather my thoughts right then and there about what to say. It also helped me recently reach the point of being comfortable enough talk on my own! Even though I still keep the journal and bring it with me to appointments "just in case."