Wow! Thank you so much, TGIG! Those are a lot of the most encouraging things I've heard thus far. It's particularly nice to hear that I'm somewhere in the middle in terms of struggling with polyamory. Like I mentioned before, I don't have much of a scale to gauge this stuff by, so I'm guessing blind when it comes to other peoples' experiences. This helps. (Also, I loved hearing even a bit of your story.)
I told her last night that I wanted to keep trying for poly rather than push it back, while I continue to work on this stuff in therapy (and do my recommended therapy exercises like a good girl, even if on paper they sound sorta...cheesy). And I already feel significantly better. Something about pushing it away, looking at it, and then choosing it again of my own accord makes it feel less frightening.
Am I 100% confident and fearless? Nope. And I imagine I'll continue to have some freaked out moments. But hopefully now that I've heard from all of you and talked more to my girlfriend, I'll be able to keep those moments in perspective, take several deep breaths, and not see it as an emergency existential crises.