I really appreciate everyone sharing their experiences with me. It's very helpful.
I felt like I needed to do something to bolster my personal sense of security, so I took on a very difficult project solo this last weekend, and got it done, so I'm actually feeling much better now - I feel much more whole and confident now. I'm still baffled at how much I just lost it last week. I am usually secure, but I really felt crazy
Girlfriend is doing better, but still a little down, but better. I think she may well have rushed too quickly into the relationship, so yea, she probably torpedoed herself, but I think thats the kind of thing she needs the freedom to work out on her own. Sometimes I have to cross my own boundaries before I can figure out where they are. Thing is that he really sounds like a great guy, really good to my girlfriend - and I don't think he was trying to "cowboy" me at all, I think he just liked my girlfriend - I can see how it might have headed there eventually down the line, but to be honest, I have a very hard time envisioning my girlfriend leaving me for someone else - I suppose it is possible; she is free to be or not be with anyone, but I can really feel that she loves being with me. In any case, I'm letting her process at her own pace.
I've learned a few things around this - my GF and I need to get better at clear communication; our personal communication styles are extremely different. I did not understand how important communication is to make this work. My GF unwittingly made an agreement with me when she was just talking about how she felt. I then had an expectation that she wasn't aware of, so when it was broken, it fueled my insecurity.
I appreciate the links to Making Peace with Jealousy in Polyamorous Relationships. The article is spot on, and has several tools I think I can benefit from.
Anyway - again, I appreciate all the feedback from everyone.