I'm sure I would be having a ton of anxiety in your shoes, but I think this letter may be less reason for alarm than you fear.

To start off with you did exactly the right thing on the Function report. Concrete examples of how your disorders incapacitate you from working is precisely what they need to have. Next you have to prepare for seeing the independent pdoc.

I saw one when I applied for SSDI back in 2012. My anxiety was awful about the interview. Turned out the pdoc they sent me to was as understanding a person as I could have hoped for. So don't assume that this pdoc you are going to is out to undermine you. That's kind of how I was thinking. Like you say, you have been diligently avoiding situations that provoke anxiety, so let that be the theme of your report to the pdoc. Explain the lengths to which you go and the depth of your fears. Again, concrete examples are illuminating.

You must feel apprehensive to get this notification more or less out of the blue. But it's probably something they just have to do, not necessarily a sign that they are looking to invalidate you. I can't predict what the pdoc will ask you, but lack of recent panic attacks won't be all the pdoc will go by. I don't think they expect you to keep seeing a therapist for years and years on end. There comes a point when you realize you have a permanent, chronic condition. The best you can do is to continue to employ what coping strategies you learned in therapy. I don't think insurance pays for therapy to go on indefinitely. It's more important, I would think, that you remain under the care of a pdoc whom you see. Sounds like your own doctor supports you remaining on disability. Think about what your doctor has said about your condition and the benefits of remaining on disability. You may not work, but surely you have other stresses to cope with. Those may be as much as you can handle. Maybe you just interact with close family. It would be okay to talk about that.

Remember: you've already convinced them that you struggle with a serious disability. If that remains the case, then you say that.