No one should feel obligated to invite someone to a holiday dinner. I think it was rather unfair of him to ask that of you. I take it that you had previously invited only him to join you and your other guests; if he accepted and had intended to go there by himself, why did he sneak that request in? Ugh. You can still tell him that you're really not able to host one more person. I hate when people pull last-minute shit like that.
Regarding your insecurities and "stages of acceptance," well, it's different for everyone. There is no rule in poly that you have to meet a boyfriend's other girlfriend, nor that you all hang out and get along. That's a personal preference -- but it's perfectly fine and acceptable to keep your relationship separate and not be expected to socialize with her, too. I get that she's cool and nice and all, and it's great to have a respectful acknowledgement of your metamour, but your relationship is with him, not her, so no need to feel like you're not "doing poly right" if you're uncomfortable with that.
I disagree with the idea that you can't control feelings. No, we can't know what feelings will spontaneously pop up, but yes, you can control them, and actively change your emotional responses to things. However, it is challenging and not easy to do so. If I were you, I would take things more slowly and ask for time to get used to this dynamic. Don't put yourself into situations that are more stressful than you can deal with -- if you had a panic attack, you need to honor that part of you that has anxieties and ask for the respect you need from him.
You're young and this is a new relationship. You are still getting to know him. You don't have to be all buddy-buddy with his gf right away and be expected to play host for her. Formulate some personal boundaries for yourself. And then learn to express them and enforce them.
Last edited by nycindie; 11-21-2012 at 10:11 PM.