She'd long known that I "wished" I could be a woman. She actually triggered my re-examining it and coming to the retrospectively obvious realization that I wasn't just wishing. We were having a conversation about sex and I mentioned wishing I could be female, and she pointed out that, you know, it is possible. As usual, I gave a few reasons why it's a nice thought but was actually not possible for me / us.
I didn't sleep that night. I couldn't stop thinking about it.
A few weeks later, on another sleepless night, it occurred to me that there was no more reason to debate internally. I was transgender, but sadly because of our kids and various other reasons, we'd have to acknowledge that and be happy with that. It seemed important to me that she know how I felt.
So the next evening I took her aside to explain that I'd been thinking about that conversation a lot, and had realized that there really was no question, I absolutely was transgender and absolutely wanted to make the transition. My expectation was that she'd be sympathetic and we'd talk about how it's sad that we just can't make that change to our lives.
Expectations are not reality, though, and the conversation did not go like I pictured it. She did a double-take at first, and hadn't realized what an impact that earlier conversation had had on me. We never really talked about why it couldn't happen. She was supportive, though a little neutrally so, but we did realize that we were in about as perfect a situation as one could be in before starting transition.
The next day, I talked to her about it some more and explained that I appreciated that she was trying not to push her own feelings about this onto me, but that I needed to know how she felt for her own sake. After a bit of prodding her, she let on that she wasn't just supportive, she was absolutely ecstatic at the prospect of the change! She just didn't want to push me to do something I didn't really want to do. She then, and still, has a little trouble believing that anyone would feel like they wanted to "change" their gender because she is unquestioningly cisgender. This worried her a little because she was super-excited about doing this, but had a nagging fear that I would change my mind, and that left her feeling a little powerless.
But . . . we are doing great, she is awesome, I am awesome, and our relationship is better than it has ever been. Our sexual relationship is different at the moment, sort of on hold (sexual activities are sort of in a confusing place for me emotionally right now), so in some ways it feels like we're just very close friends rearing kids together right now. That's actually ok with me, though I think we'll move back toward feeling more like a married couple when I move farther along transition and start to feel more at home with my new feelings and physical changes.