I know that it's very hard to put your finger on your thoughts but my therapist said that when you are more mindfull it will become easier. I find it very difficult but lately I have been writing down something that has annoyed or upset me. It may only be a word that you write down but it helps so much and by helping yourself in this way it makes it easier for the therapist and maybe have a more meaningfull session. I am no expert but I think I have to help myself.
Best wishes