Just an update:
The mental barrier has been crossed.
Basically, the issue ended up being that I was still presenting as male at work. So even though I didn't like that presentation, it was still the "default" self that I saw in my head, because it's the self that I interacted with the world as. So male genderings and being treated like a guy were the default, while female genderings and being treated like a girl were like a special treat that I maybe got once a day if I was lucky.
About a week ago, though, I was fired from my job. And as much as that sucked, it allowed me to play around with this "default" presentation. I played around with my hair style, and finally discovered a wigless presentation that people read as female by default... with my hair tucked back into a ponytail, and wearing a feminine pair of dangly earrings. With that, I'd finally found a definitive "girl mode" that took no more effort than my "guy mode," and because it didn't involve a wig, I felt like it was WAY more incognito, and therefore didn't make me stand out as much. Basically, I was completely comfortable as this self. And this self was still being gendered female pretty much 100% of the time.
I started dressing up as this self every single time I went out, and before you know it, just like that, my default mental state re-wrote itself so that now the self that I was seeing in my head was this ponytailed female self.
And I started thinking of changing my name, not because of it being some grand event that officially changes my life forever, but rather just because every single time I'm having to tell store clerks that I have a male name, they give me a funny look. And the gender marker change, again, isn't the act that's asserting my femaleness, it's just to match my legal status with how the world perceives me. Basically, the key was to quit viewing transition as this scary distant thing that would turn me into a girl forever, but rather to start seeing it as something that just legally recognizes what I already am and what the world already perceives me as, and the self that I always appear as anyway.
So yeah... that was the key. Just finding a "girl mode" that I was comfortable with, and that I started being comfortable seeing as being "me." When I started this topic originally, I didn't have that. I just had a "girl mode" with a wig that always made me nervous when I went out because I was afraid I stood out too much, and a wigless presentation that got gendered female maybe about half of the time. So I didn't feel like my "girl mode" was actually me, I just felt like it was a way that I dressed up now and then. Where now it's not. It's just who I always interact with the world as, who I'm comfortable as, and the legal proceedings are basically just a technicality.