For me it wasn't just one thing. I was going nuts, highly depressed and becoming highly aware of my lifelong gender issues. After I woke up in the hospital the second time inside of a month, I confessed my issues to the plastic surgeon who was stitching up my head. It turned out that he had quite a bit of experience with transgender women doing minor FFS procedures and BAs. He told me that I needed to see a therapist who specialized in gender issues right away before I drank myself to death.
A month later, the therapist I found had me pegged as transgender within 10 minutes and told me a bunch of other things that I didn't want to hear, like "You need estrogen", "Why on earth do you think you can't transition? You seem like a perfect candidate and it's not that big of a deal." and "Let's get you set up with a psychiatrist and an endocrinologist."
I almost chickened out, but I saw the endo in the end and got my E-ticket. Part of me hoped that the estrogen wouldn't do anything at all, or even make things worse, thus proving I wasn't actually transgender and I was just a standard issue nutjob. That would have been much easier to deal with in the end, but the E was like the magic bullet for me. My head was the clearest it had been since before the testosterone poisoning began, and I was (OMG!) actually happy. I didn't know that one could even be this happy. It became evident that I could never go a day without it.
I started laser right away and began to test the waters very slowly. One day I was able to go out my front door in girl mode. A week later I was able to get in the car. The next week I was able to go to therapy in a dress, and the next weekend I mustered the courage to go to San Diego. I was pointed at, laughed at, stared at and ridiculed publically, but it didn't stop me. I just worked harder on my demeanor.
I started going out to dinner in my own neighborhood and it got easier every time. Within a couple more weeks, I noticed that there were guy clothes I could no longer wear because my nipples were getting rather, umm, pointy. I began to feel ridiculous in guy mode and realized how completely fake it was. I stopped caring about what anyone else thought of me and I just began wearing whatever I wanted to wherever I went.
About a week later, we went to Vegas for a wonderful 3 day weekend. I intentionally left the boy clothes/safety net at home. It was "sink or swim" time. It was a little bit nervewracking, but I was actually for the most part, passing. I decided that weekend to make it all permanent. I hatched a plan and came out to the world. I told my therapist that I had gone full time, got my carry letter and had my HRT doses upped by the endo.
TL;DR- It wasn't one thing. I just followed the path of least resistance.