TransandSo,
Wow, what a difficult spot to be in. I can understand both your questioning and your guilt. I am so sorry, it came to you as if out of no where. I am so sorry.
I have a relative that has a history of mental illness that this feels like, from your description. One always wanted to support him and help him through. However, he would self diagnose and alter medications till he was back in a hospital again.
Being in the unique situation of personally understanding gender dysphoria must be both a blessing and a curse. You mentioned professional diagnosis. Is he currently in therapy? Are you involved as well(couples or able to be heard by the therapist)? Seems like that would be helpful; I just don't know if it is an option. Are you currently in therapy? It strikes me that would help you sift through your feelings, of pain, betrayal, love, support and just being tired of it.
I am sorry that I can not really be of help to you. Except to listen. I am a 50 year old MTF. I have been married to my wife for going on 25 years and we have 3 teenage daughters. I guess I can understand arriving at a realization pretty late. It only really put it together this past Feb or March. I felt terrible coming out to my wife. She has been very supportive but I am sure she has days similar to how you feel. I have been "miserable" to use her word for about 3-4 years. However, I have not been quite to the depths your husband has sunk. I was just depressed, anxious and unpleasant. January was the first time I had been to a therapist.
I hope you find a clarity and wish you a smooth path to follow.
With warm wishes,
Joanna