Have you been released by your doctor to return to work? If you're on temporary disability until January, I'd certainly say not. So they probably will contact you to ask if you're released and ready to come back, at which time you'd simply say that you have not been released by your doctor. Or maybe they'll just write you a letter without discussing it with you, telling you that you've been discharged due to your inability to return to work after your FMLA ended. Either way this is a discharge due to health reasons, not a discharge due to misconduct.
Look, no one generally chooses to deliberately become ill or are able to make themselves well on demand. What you have is a verifiable mental health issue, which makes it impossible for you to do the kind of work you have been doing, or any work, right now, for that matter. So my dear, at this age and stage of the game, forget about "marks against your RN license." (you know, that MIGHT happen if you were to go back to work and have a panic attack or mess up a treatment, or run out of the place screaming....might....) They're certainly not going to give you demerits or marks for having a disabling condition. That you have sought treatment for it and have made the decision to follow the advice and treatment plans of your doctors is the only reasonable professional response you could possible have.
Though I'm sure your RN licensure has been a very important thing to you in your past life, you're, at this age and state of mind, looking at not exactly doing this anymore. I'm older than you, and have had many health issues, but I can tell you that looking at a new part of your life is something that is coming up. You sound extremely burned out and in need of rest and recuperation. And you're still worrying
! Perhaps later, if you get better, after your life is worked out in other directions, you can do some part time or pick up work in the helping professions.