I feel the same way quite often. For me, it just took time. I couldn't force myself to trust my T, and I still struggle greatly with it. But it does get easier. Eventually, you will be able to trust a little more, and a little more, and a little more, until you have the ability to be really open. Even saying what you did showed a lot of trust and risk, and it shows that you are improving in your trust with him. But it will take time until it's not so terrifying. You're in one of the hardest parts of therapy: just starting to really trust, and all the terror and confusion that comes with that.