I suppose I became comfortable with my gender when I was able to separate my biology, expression, and identity from one another.
Biologically I'm male, which doesn't "match" my preferably feminine expression, so I used to think that changing the former would make me more comfortable with the latter. However, that was rather troublesome since I had no real ambivalence towards my sex organs. If fact, I rather enjoy my junk and don't want to be rid of it at all. So, I began to think of gender as a spectrum instead of separate and opposite binaries.
Then I struggled to find a point on that spectrum which I could label as my "gender identity". This is basically impossible since my gender expression is always the same and my biology is constant. It's like a math equation: if one variable in the equation keeps changing while the other stay the same, the answer will always be different. Thus, gender is fluid, and I don't try to label it anymore.
I tell a much shorter and less eloquent version when I'm at a bar and incapacitated, which is probably where I get asked the most. So now I'm just going to right this equation on slightly stained drink napkins:
x + y = z (where x is biological sex, y is gender expression, and z is gender identity)
x + 4y = f (where x is biological sex, 4y is a pair of 4-inch heels, and f is fabulous)
My y is always changing. Can't help it. And I'm fine with my equations never exactly balancing out.