In my case, it was a journey.
Up until 2+ years ago, I'd always thought of myself as a failed man, or, later on, as a gender non-conforming man. I was a guy because of what was hanging from my crotch, but my connection with my "fellow" men ended there. I didn't think like them, I had no interest in what men seemed to be interested in, when I interacted with men, they seemed like an alien species that I had had to consciously learn to interact with. Whenever discussions of women's experiences came up, I found myself identifying with the women, not the men. I was a hardcore, even radical feminist long before I knew the word "feminist." And the way I felt and responded around sex was a lot more like what they say about how women see sex than men.
When I got divorced, 10+ years ago, I swore I'd stop trying to be what I wasn't and instead try to find out who I was underneath all that ****. I started wearing skirts and hung around in (on-line) men-in-skirts groups. But in contrast to all the men who were intent on proving their masculinity (despite wearing a skirt), my urges led me to present more and more "femme."
Then I read Zinnia Jones' blog post
"That was Dysphoria?" and went, "that sounds a lot like me."
That led me to Susan's, and over the next six months or so, I realized that seeing myself as trans allowed me to make sense of most of what I hadn't figured out about myself. And then I started considering transition, and when I did, I realized that I was going to transition (that is, that is where my life and whatever it is that guides it was taking me), so I might as well make the best of it. You know, it was like the "me" that isn't my conscious mind but is what really runs things said, "we can do things the easy way -- or we can do things the hard way. Which is it going to be?" Me, I'm lazy. I'll always opt for the easy way.
I suppose I could try to "decide" that I wasn't trans. But it would be like "deciding" that I'm 5 foot 2 and have hair down to my butt.
There's been a lot of pain in my life. But accepting that I'm trans and going forward to live in a way that better fits who I now know myself to be hasn't been painful at all. Scary, yes. Complicated and a pain in the you-know-what, yes. But the further I go, the less I feel like I'm being torn apart and shredded. And that, to me, has been the firmest proof of all.