I hope it will go okay today and you'll be able to say everything what you'd like to.... I often have a similar problem, but usually I e-mail my T between sessions with some feedback and comments so when I come very defensive to the session she starts with the e-mail and I have "no choice" and have to discuss what I actually wanted... If my defensive teenager is active (is it always a teenager?), I just say e.g. "sorry but I'm very defensive today and I totally do not listen to you and just want to leave" and then usually my T knows how to solve it...