During the time I've spent on poly forums, I've observed that it's not uncommon at all for someone to have a fear of being replaced, or to have emotional difficulties when their partner "falls in love" with someone else. For some people, their partner having sex with someone else is what bothers them. For others, it's the emotional connection that seems more threatening.
Some things get easier with time. But you should persist in communicating your feelings, whatever they are.
Even if it is awkward, you might want to spend a little quality (platonic) time with Hubby's girlfriend. Such as a girl's night out, or that type of thing. Sometimes having a better friendship with someone makes them seem like less of a threat, and they feel less threatened too. Yes, this girlfriend may be afraid you'll pronounce the ultimate veto, and kick her to the curb. That doesn't mean it's a rational fear, but some fears are irrational. Being in a V situation, I've found that it does help to spend time with the other "leg" of the V.
Love means never having to say, "Put down that meat cleaver!"