Thanks Julia for the response, I'm glad that my opening up helped you and possibly yours as well. The world is always made a better more beautiful place when we can find understanding and compassion for each other's moments of fear, grief, or weakness
As the "woman melting down" if I could offer any advice to the "OSO" to help every one be ok, I can tell you what I think would help me, if I were in her shoes...
First and foremost she is going to have to work on her "stuff", whatever is inside her that has triggered fear and insecurity, and no one else can do that for her, it's true... but both you and the shared partner can do so much to help her feel safe and secure and supported in her healing and self-discovery
She likely needs reassurances, lots of them, that she isn't going to "Lose" anything or anyone in this situation of sharing, and perhaps she, and all of you, can gain... more love to go around ya know. I don't know you're situation, if you and she are friends or know each other... but if you're not, that would probably help. I've been thinking that I would feel more secure with other girl's my Lover may be interested in, if I really got to know them, and developed my own friendly (or maybe even playful sexy) relationships with them, and then I would know that they A. respect our primary relationship and aren't trying to take him from me, and B. that I know them as real persons, not as imagined in my fears... which is always worst, the fear of the unknown.
And the shared partner can be supportive, loving and reassuring, while still keeping balanced boundaries... depending on all of your agreements.
Since I found this site, and started reading posts for my own situation, the one thing I've seen stated repeatedly that made the most sense to me is "Take things at the pace that is comfortable for the person who is struggling the most, have compassion and care about the hearts and feelings of ALL of the Loves involved."