YESS! I know that!
There has been a time when I ALWAYS left the therapy with a clear knowledge that I had left important things unsaid. That was year 1 and maybe 2 of therapy.
For me it just took time before I solved the problem. Now T is way more fruitful (year 5). Uhm, maybe you should really WRITE DOWN what you have to say before the T. Then you immediately take it out and say it.

I clearly remember that at one point, being frustrated by yet another depressive episode, one day I entered T, sat down and said "the last thing I'd like to tell you is..." and I revealed the MOST embarassing detail about me. It didn't make any particular sense, but I broke a taboo. The therapist was ready to work on the issue.

Do you think you could try this?

The best of luck