I really think you have to trust yourself, you have to trust your therapist and you have to trust the relationship. Crying in front of another is a very vulnerable act and puts you in a vulnerable position with your therapist. You have to let him/her see you in a weakened state. When the walls come down you have to know that your T will accept you, love you and take care of you. And you have to allow your T to do that. That takes a whole lot of trust. It's also risky knowing that letting go may make you feel closer to your T. That's where trust in the relationship would be important. So I do think that trust is the key. When you feel ready maybe vulnerability will come easier for you.