For the record, I thought Mercury was making some really good points about cattiness between women, until she started insisting that it's easier to be happier in monogamous relationships.
But now, to redirect this thread back to advice for Fiona:
My first reaction to your situation, Fiona, was to agree that your husband's girlfriend sounds kinda awful. I started to think, yeah maybe this is what vetoes are for.
But then I thought about it more (aside from not believing in vetoes anyway). It seems like the issues you have are with your husband, but you are getting really distracted by the girlfriend and/or blaming the issues on the girlfriend.
For example, when the girlfriend forgot something at home, it should have been your husband's job to say, "Sorry, I don't have access to the car." It's not that the girlfriend was being dramatic or creating drama--your HUSBAND created drama by not respecting your time with your boyfriend.
Maybe she IS dramatic and immature. But it sounds more like she is just a bit inexperienced with poly. The comment about giving your husband a blowjob, the awkward things she said to your boyfriend--those don't sound like malicious comments, but like the attempts of an uncomfortable person to figure out what to say.
I wouldn't automatically think it's not okay to joke about giving a blowjob in front of one's metamour. I mean, some people would be fine with it. Is she supposed to pretend she doesn't have oral sex with your husband? Sure, it's awkward to mention it, but treat it as her awkwardness, not as her immaturity.
If she really is immature, you want to know what your husband sees in her. My knee-jerk reaction was, "Ew, I hate it when guys get caught up in sexual feelings for an immature woman and think they have real love." But part of my reaction comes from a lifetime of absorbing catty attitudes toward the "other woman."
Maybe she's a bit immature. But the fun thing about being poly is that you CAN date someone who is not the type of person you'd pick for a primary partner--someone who is totally different from the primary partner you already have.
One of my best dating experiences as a single person was becoming involved in a casual/sex-friendship thing was a guy who was TOTALLY unsuited to me. He was a total flake, he was irresponsible, he smoked, he had wildly different political views, he was too old for me, he was...frankly not all that bright
Oh man, was he fun. I learned so much about myself and about sex. In many ways, he was a pretty good friend to me too.
Part of the time that I was involved with him, I also had a boyfriend/ex-boyfriend that I was non-monogamous with. I liked my boyfriend quite a lot. I would NEVER have expected that he would like the other guy. (Because of distance they never had to meet). I talked openly about the other guy to my boyfriend, and he was supportive, but if they had met I'm sure my boyfriend would have thought, Whoa, that guy is flaky and immature and what the heck does she see in him?
So what I'm saying is, maybe let your husband carry on his relationship the way he wants to do it. It might end eventually for reasons that have nothing to do with you. In the meanwhile, give him the freedom to date as he chooses. You don't need to like her.
However, that said, I see some big problems here. With your husband, not with his girlfriend. I am REALLY bothered that your husband said, "Well she doesn't like you either" to you. That is REALLY harsh and dismissive of him. And hurtful.
The issue is not that she doesn't like you, it's that your husband doesn't seem to like you much right now either.
No wonder you are feeling panicky and unhappy.
It sounds like you and your husband should work on your own relationship a bit (dates nights, etc). Maybe you should flat-out not discuss the girlfriend with him since maybe that's only part of the issue.
Actually, I think you should try harder to communicate with the girlfriend and get to know her better. As an introvert myself, I know that's hard. But it seems like she tried to reach out to you but you shot her down, and now she's struggling to figure it all out.
I agree with Mercury's points in some ways: we all really do have a knee-jerk reaction of jealousy against the interloping, manipulative, immature other woman. You're even trying to assemble evidence from her other partner's wife that she might be "trouble."
It sounds to me like she's trying as hard as she can, but she's a bit clueless.
Also, can you and your husband buy a second car or something???? That arrangement sounds ridiculous.