I quite often can't talk in sessions but it's not the end of the world since I have body psychotherapy. When you enter adult services maybe you could try and work with someone who utilises the body. Even if not for me the solution has seemed to be to stay with it. Occasionally these days I can talk for about half an hour but that is after 3 years of therapy. It sounds like your therapist is patient with you and understanding so I would just stick with it. I have found that if there is something I really need to say I email it before. I still can rarely talk about it but then she knows the problem which helps.
Try not to be too harsh with yourself. I think this can get better but it won't over night and that's ok. I think I understand your frustrations and I know you don't want to hear give it more time, but from someone who's been in your position you may need to. There is probably no quick fix but there are other ways to communicate. Sometimes if I am very stress she gets out vast sheets of paper and pastels and we just draw. It's amazing how much you can communicate this way and for me drawing is just random lines and squiggles not actual representations of things. I'm sorry this is so tough I hope others may have other suggestions. I sometimes sit there and work out how much money I am wasting each minute of silence! But it is getting better.