Well, you're still thinking from a monogamous mindset. You're OK with your bf having casual, sexual relationships, but not fully emotional ones. This is very common with poly newbies. Generally some are OK with emotions as long as there is no sex, or sex as long as there are no-- feelings!! Eeek! Feelings!
When you understand poly, you will learn that if your partner IS poly, he can have emotions as well as lust for more than one. If he was wired mono, he'd leave one gf if he fell hard for another. But since he's poly, he can love you deeply while still falling for another.
It's too bad you didn't go to that party and meet her. Quite often, meeting a metamour cancels out much of the insecurity. I'd recommend planning a date ASAP, if she's willing, with either just her, or with your shared bf. Coffee, a drink, a walk, an hour or 2, short and sweet.
If even that seems too much, have her come to the house to pick up your bf for a date and at least chat for 15 minutes as a start. Face your fear. It's good for your health. Be scared, but do it anyway. The world won't end.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37