A final post:
We talked it all over. She decided it wouldn't work. She wants to be with him and sees polyamory more as a want than as a need, at least right now. She promised me that if it ever becomes a need she will ask him to deal with it and he will, but right now she doesn't feel the drive to have someone other than him. He makes her very happy, they're partners in the truest sense of the word, and she quelled all my fears about her relationship with him.
She also told me that despite her calm in front of me, she cried when she got home after our first talk. She just doesn't cry in front of people because of her past. It made me feel secure to know this hit her almost as hard as it did me.
I feel at peace. That this is her choice and that *she* feels more or less at peace about it is really important to me and makes it more final and easier for me to move onward and upward.
I asked her to tell me if anything changes with regards to her needs or if her relationship ends and she wants to date again. Then if I still have feelings, we can try something.
Otherwise, radio silence for now, until I stop missing her and until I can go a whole day without thinking about her. Then, slowly, friendship. Yoga together so I can work on tolerating her presence without getting tense or feeling conflicted - focusing on myself more than on our interaction, but just being with her.
Reminding myself of the few things I don't find amazing about her - her insecurities about her body, her insistence on following "hippie" practices she doesn't really believe in, her flightiness / fear of commitment (just less so with her current boyfriend) - these things will all make me feel better about not getting to be with her.
I met a really great girl tonight, and I'm going to ask her on a date. It will be a good start.
Far better than that, I got a new kitty today. I didn't know I'd be getting him until yesterday afternoon, and I got an email about a kitty needing a home and I thought about it for about fifteen minutes and just said, "Yes." He's one of the best kitties I've ever been around - purring, playing, cuddling, and he's FLUFFY. It will help fill this hole.