LB73 I feel your pain!  I have 3 kids, married 16 years.  I knew for maybe 10 years he had a thing for women's clothing when we were intimate, but I realized that I just hated it so he stopped it.  I hoped this strange urge of his went away, but then 2 years ago the phrase "maybe I'm trans" came out of his mouth and I held him while he cried wondering if his parents would ever accept him as their 'daughter'. 

I'm not entirely sure why I didn't run then, but I guess I wasn't ready to look at the facts clearly at that point in time.  I think we all have to go through a process to be able to see and accept reality and make a decision on how to move forward authentically.

Individual counseling helped me immensely in thinking things through clearly.  For marriage counselors, be careful as some have an agenda and automatically side with the LGBT spouse and feel their job is to get the straight spouse to jump on the trans train and go along with everything.  That happened to me and I insisted on finding a new, impartial marriage therapist.

Another thing that helped me was setting boundaries.  I am straight.  Not a lesbian.  Not bi.  I am only attracted to men, period.  So for me to be my authentic self in a marriage, I need to be married to someone who is physically male, not someone who is gradually changing his body into that of a woman.  My boundaries (for me to stay in the marriage) were:
1) No surgery (to feminize)
2) No hormones
3) No dressing as a woman in our town (~150,000 people, in a conservative state, small enough that word would eventually get around), but dressing up when out of town was OK.

Another thing that helped was talking to a couple of childhood friends and a sibling who live on the other side of the country.  They kept our secret as to what was going on and it was an immense relief to step out of the closet I had been shoved into and find genuine compassion and support.

If you are too stressed out to think clearly and your husband is pushing to move forward with being trans, ask him to pause his activities so you have space to process.  If he won't give you time, then consider a separation.  I was genuinely worried I'd end up in some sort of in-patient mental health facility and realized I needed a separation.  We rented a tiny, cheap apartment and my husband and I worked out a schedule so one of us was always at the house with the kids while the other was at the apartment... we didn't talk to each other much except about the kids and their schedule.  And some evenings we both had to be on duty driving kids to various activities.  Each of us had at least one weekday evening and one weekend day to ourselves at the apartment. I hadn't had this much time to myself in a decade and it was beautiful!  I used the time to journal, pray, listen to Straight Spouse podcasts, call friends, go for long walks, and read all the books I could find on marriage, transgender issues and especially stories of women in my situation.

I think this time of separation helped me rediscover my dreams, hopes, desires and authentic self.  We went through several months of intense efforts to rebuild our marriage after our separation, but in the end my husband decided he could no longer work on our marriage or abide by my boundaries.  So after 1.5 years of intense effort and immense agony, we're in the process of getting divorced.  For me, I felt I had to try everything to save our marriage because of my faith and our kids.  Now I am confident that I can look my kids in the eye and tell them that I tried everything, but could not stay married to their dad.  I think having divorced but happy parents will be better for them in the long run than miserable and bitter parents.

It takes time to honestly evaluate your marriage, reconnect with yourself and figure out how to move forward.  It can be painful and slow, but so worthwhile in the end.