Don't feel bad, I almost did the same exact thing...
We hired a new woman recently, and a coworker referred to me as HE and HIM to her a number of times. Naturally, I assumed I'd been exposed, and I've been SO tempted to ask her if she'd figured me out BEFORE I was exposed (I'm still a BIT insecure about my passing ability, lol).
But I've spoken with her since then, and I swear she still doesn't know. I kept thinking back now about how awkward it would have been if I'd started talking about it, assuming she knew, only to realize I'd just outed myself.
Sooooo....
My rule is to NEVER bring up the subject myself. Never. That's terribly difficult for me, as it goes against my nature to not explain myself (more insecurities), but it's served me well so far. If someone asks politely (hasn't happened yet), I'll discuss it - to a point. But no justifying, no mention of "birth defects," or transsexualism, or GID, or being born this way, or... whatever. No, just "Yep, I transitioned a few months ago and I'm Kate now." IMHO, the more we "educate" people, the less normal we seem to them. The more we describe ourselves as having a "condition" or "birth defect," the more different and odd we seem. But if we focus on the facts ("I'm xxx now"), people seem to take it more in stride, as you give them nothing to question or argue with.
~Kate~