Do you think maybe you held the death of the relationship inside your heart all those years when you were dead inside at the sessions.? ISometimes if the pain is too deep, we put a numbing emotion ontop of it. When you knew it was the last session, after that there was not a chance to have to see him and have him see the anguish in your eyes, your subconscious may have known
it was now safe to process through that death event from all those years ago.

After you processed through, you could feel again. But you feel right at the worst timing of all... after the relationship has ended.

You deserve to be heard about the relationship and the ending. Now I do think talking to another therapist is a solid idea, I also understand the need to be heard by YOUR therapist. I highly suggest you write out everything you need him to hear and mail it to him. He may or may not write back, but he will read it. And you will have said what you needed to say.