Congratulations on starting therapy and coming out to your wife! Those are both big steps.
I am sure there are good articles online, but I don't have any hand links to share. No doubt other members will. I got most of my information right here on Susan's Place.
When I was coming out to people, I avoided using the word transgender right off the bat. I would tell them that I had been diagnosed with gender dysphoria, which is the term that the medical professions use for people that are transgender. (And that was the only time I used the word.) And then I would explain that gender dysphoria meant that there was a mismatch between the gender of my body and that of my brain that had existed since I was born. And I made sure never to say that I "wanted to become a woman". Instead, I said that I wanted to live as the woman I was born to be.
The key points there were:
- it is a professional diagnosis (so it is not my imagination)
- it is a physical condition (not some kind of delusion)
- I was born this way (so it is not something that "happened")
- I was born to be a woman (so it is somethingthat I need, rather than want)
My thinking was to take away, right up front, the possibility of them thinking this was a choice or something kinky. It worked pretty well for me. Perhaps it will help with explaining it to your wife.
I was 61 when I had the talk with my wife. So of course, the "Why didn't you tell me sooner?" question came up. And I told the truth. I had often wondered if I was trans, and that I had always managed to convince myself that the answer was no. So I didn't really know, myself. What changed was that I could no longer convince myself.
It might be helpful for your wife to have a session, by herself, with your therapist, so she can ask her own questions and get answers directly from the them.