Yeah, It happens to me. I think it's a normal human behavior. You will probably would open more if the person was really good at their jobs. You should just try to get your mind ready before the session to say whatever you want without filter. And don't worry afterwards what he may think, he's a paid professional. He has probably heard about everything there is to hear about other people talking about themselves. If you really want to test how good your therapist is, is by talking all that he asks of you without filtering. And by judging their reaction you can tell how good of a therapist they really are.
Just remember that is normal, and just get the mindset ready. Trust me it works. 
If your goal is to open up more to your therapist. If you don't want to feel vunerable you can detach your self emtionally and just say the things as they are.
Absolutely! (and it's hard) but that's the point to open up and expose. The therapist will be able to help you more. It's okay and a good thing to expose yourself even though it may be uncomfortable and embarassing. It will help you in the long run.
as if my mind is ever right) & I told my therapist way too much. I was just being honest & wish i could do that again, but my experience with how she reacted to my honesty made me wary of ever doing it again. I've been seeing another therapist for almost two years now & I've told her virtually nothing about myself, at least nothing really personal, because I'm afraid of her reaction. I'll be switching to a new therapist soon & I'm not sure what the experience will be like, but I'm hoping it will be better. I wish I could find a therapist who truly is nonjudgmental & who would just listen to what I have to say without contradicting me. If I never find that, I don't see what the point is in going to therapy because it will never help if I can't be open with the person. 