I could go from the beginning of my life to when I decided to transition and tell you all the breadcrumbs along the way that got me here, but you're specifically looking for what sealed the deal. That is surprisingly tough for me to pin down.
I found out the word "transgender" when I was 17 after a life up to that point of a lot of discomfort and confusion. I spent months researching what that meant, what transitioning was, and what all could be done. I knew, absolutely, 100% at that point in time that I was transgender. There was no doubt. Unlike the labels I had clung to at earlier points in my life (tomboy, lesbian, butch, etc.), this one completely made sense to me. There was no aspect of it that did not fit. I poured over as many blogs as I could find, talked to my friend (who had come out to me and was the reason I had discovered the word in the first place), and got introduced to other people who identified that way. By my 18th birthday, there was no doubt in my mind that I was absolutely transgender. Sealing the deal in the sense of accepting and identifying as trans for me just took social confirmation. I needed to hear other people's stories and feelings and have them be similar to my own.
Yet it would take me another 8 years to transition. I was absolutely sure of my identity. I made small adjustments in terms of my presentation, tried on some different labels to see if they fit (thinking perhaps I could be happy with androgyny). It took reaching my 25th birthday and realizing that I had lived what will likely be more than a quarter of my life as someone else to really seal the deal. The internal me didn't match the external and I could no longer mentally carry on letting that be the case. I was tired of lying to people, tired of being uncomfortable with myself, tired of my relationships suffering, and tired of putting off the only thing that I knew would help, which was transition.
Now, largely post-transition, I am absolutely sure I did all the right things for me. It's easy to say that looking back, but I was totally terrified along the way. I think the vast majority of us enter into transition with some degree of uncertainty, whether that's with being trans in general or what we need to do in terms of transition to get to a point of stasis.