This happens to me. But I fall back on the idea that a therapist bases what they talk about and do in sessions on each client. If I want to have a good to great therapy experience its up to me. The best way to do that is to know why you are in therapy and what you want to accomplish and then go into the sessions with a goal.

So the night before I attend my sessions I read my journal entries and sometimes choose a few to take with me, I also draw so I look through my drawings and see if there is any drawings I want to take with me. I also do workbooks on my own so I look through that content to see if there is anything I had a problem with doing or still is hanging in the air that my therapist may be able to help me with. If I find a new book I take that also so that my therapist can see that. I also do craft projects on my own around my problems and have from time to time taken in a diarama, a collage, and so on.

Anyway I always have "something" in my back pack that I can pull out for those times when I can't seem to talk.

And if all else fails my therapist and I do relaxation visualizations during sessions so I can always say to her - I don't feel like talking right now lets do the relaxation stuff.

Hang in there. As you have more sessions talking with your therapist will get easier. You will probably always have times when you can't find the right words but at least there won't be so many of those times.