I think it's really really clear - to me at least - that all of my insecurities have nothing at all to do with C. At a really fundamental level, my jealousy is not 'what if she's better at x/prettier/smarter/funnier than me?' but 'what if he prefers her for x?' - a subtle difference but an important one, I think. I also think that I'm probably projecting quite a bit onto their relationship - I'm pretty sure that what she's after is a comfortable FWB situation with both of us, and would definitely not be interested in getting involved in all this drama! It wouldn't be fair to her, as it's not about her. However, I feel like I do at least need to address all my worries now, rather than further down the line.
I suppose it's just readjusting; we've previously always had very compatible life goals, and he seems to have changed his mind this summer over what he wants. That's not to say this is a bad thing; we ought to be able to develop and change together, and I certainly wouldn't want him to stay exactly the same as he was when I first met him. it's hard to adjust to, though. And it feels a bit like I'm being left behind.
I am a bit concerned though - he thinks the reason I'm getting so worried about all this is that I'm overthinking it, and we should just carry on and see what happens, and not talk about it so much. I think we need to talk and talk and talk, because it's so important to me that we take care of our own relationship, rather than possibly testing it to destruction and not realising until after the fact.
As for meeting local poly groups... yeah. Funny story, that. We have quite a few friends who are poly/non-monogamous/swingers/other, but I've had arguments with my partner over the fact that he is/wants to sleep with ALL of them, which leaves us with NO friends involved in all this that we aren't also sexually involved with. And hence impossible to talk to about my own relationship worries. I get frustrated by that; it takes a lot for me to find that 'zing' - someone can be interesting, attractive, funny, whatever, but there's that indefinable something for me to want to have sex with them or not. Whereas for him, his criteria is basically any woman who is intelligent and good-looking. (Admittedly, that's better than some people!) I suppose that's another thing I need to adjust to.