I am glad to hear that you have finally started your CBT. Since this is your main concern you should really address it next time you see the therapist.
I am so proud of the courageous effort you are making to retrain your mind to realize that there really is no harm to your neck or heart when you do this movement.
You do realize, at least on the intellectual level that you sometimes get that anxiety
reaction of shortness of breath or rapid heartbeat.
By this time you know that it is purely an anxiety reaction, and one that you will soon learn to overcome.
I did think of something that you can try. Go through the same motions of looking down and then looking up, but this time, keep your eyes shut. If you feel comfortable doing this, then keep practicing.
You then next might want to graduatge to a blindfold and do the same thing. You don't have to do this at the same sitting.
If that works, then try dark sunglasses in a darkened room. Try first with your eyes shut, and then with them open. If you can do this successfully then try increasing the illumination of the room slightly at the next session.
By the time you get back to the therapist you can tell him or her of your situation and how you have been practicing exposure and prevention techniques on your own with a measure of success.
I am hoping for the best for you. Continue to be brave.