A lot of good, fundamental advice you've given and I certainly wouldn't disagree with any of it. I think we both strive to meet and/or exceed in this area.
As it pertains to this situation, I have in the past not attended a birthday party because I knew the Ex would be there. I don't always know, and have avoided social situations with the best friend because of even the slight chance. Soon, the best friend is having another party, and there is another chance the Ex will be there. I am once again faced with the decision of not attending as part of a couple, face the inevitable resentment from my fiance, and the repercussions of that. Or, attend and be upset, and resentful myself. We have been invited to attend this next party (actually its her best friends son's 3rd birthday) and my absence would be noticed. It was easier to avoid in the past when our relationship was still young, but the more we become a couple that does this sort of thing, I'm expected.
In the past when I do talk about my feelings on the matter, and I've been very clear about it. It's not uncommon that at the end of the conversation I'm accused of being harshly jealous and insecure, that I need to just 'get over it', and further I received many links to internet web sites discussing being jealous and insecure. My goal for coming here is to accomplish a few things 1) Determine if my insecurity and jealousy, in this situation, is outside the norm. I understand everyone can be a bit jealous and insecure at times. But am I at a point of ridiculousness? Do I really need to just 'suck it up' and deal with the uncomfortableness. Or do I have a real case to stand my ground and continue to avoid contact. 2) If I have a solid case and my level of jealousy/insecurity is nothing out of the ordinary, then I am open to advice on what to say to her about this upcoming party. My last suggestion to her was, was to talk to the best friend and tell her about how uncomfortable it makes me, and the two of them could work to help avoid the uncomfortable contact. Maybe give us a heads up if the Ex would be there, and at the same time, being let in on the issue would minimize the possible insult I give by just not attending their social gatherings with some made up excuse. My fiance didn't like that idea, and would rather not bring it up with the friend as it would embarrass her to disclose my ridiculous jealousy and insecurity.