Terri,
Welcome! This is a wonderful, supportive place with an incredible amount of useful information and help. You've come to the right place (just avoid the politics and religion discussions - all support goes out the window there.... LOL).
As an older transitioner dealing with a marriage and kids, I can tell you that it is difficult, but not impossible. And while you may not stay together, being honest about your feelings early on is the single best thing you can do. Yes, it's very difficult and scary - you may want to get some help from a couples therapist who is conversant with trans issues to help with this process, but the hurt, pain and resentment that your partner will feel if you hide all of this from her will be much worse and much more damaging in the long run.
If you've not started seeing a therapist conversant with transgender people yet, get yourself to one very soon. You need to sort yourself out and figure out what you need to do, or at least get to a point where you can articulate how you are feeling to your spouse.
Just know that you are not alone, that your feelings on this are valid and that you can make it.
Best wishes on your journey!
Hugs and warm thoughts,
Amy
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Life is too important to be taken seriously!
"Pain is inevitable, Suffering is optional" Ana_Okie