It is understandable that you feel anxious. There are ways to help you calm down.
One, try deep breathing. Take a very deep breath, let it out and do it again. Each time you let it out, try relaxing your body along with it.
Two, visualize letting out the tension with the air you breath out. Let it all go. Tell yourself none of it matters right now.
Three, when you see your therapist, let her know how you feel. By telling her, you take away the power the fear has over you.
Four, try seeing your therapist as human. Most people in therapy tend to view their therapist as an authority figure and while that is ok, seeing her as human helps make the power between you more even. She is there to help you, but she makes mistakes just like everyone else. She is not perfect. She has problems just like anyone else. She may be able to provide answers to problems you have, but she is trained to do that. She doesn't just come by her knowledge without help. By recognizing that, it helps you feel less anxious.
Five, relax while you are in therapy. You do not need to worry about what you feel or what you are saying. Your therapist is there to help you, not hurt you.
Six, you can always take a break. If you feel overwhelmed in therapy, you an leave for a few moments. Go to the bathroom and take a deep breath. It will help you focus again.
Let me know how it goes,