BaggagePatrol: please stop blaming yourself and feeling guilty. Your feelings sound quite reasonable.
The first thing I would suggest is scheduling date nights for just you and your husband. You don't necessarily need to go out anywhere, but you could. And these should be nights where you talk about yourselves and each other, not your girlfriend.
The idea would be to generate more excitement and connection with your husband. It must be REALLY annoying to see him staying up to midnight talking to the girlfriend when you know if it was just you and him he'd crash and go to bed at 9pm.
Also you should schedule time for just you and the girlfriend to bond without your husband there.
And then the husband & girlfriend can also have a night to themselves while you pursue something on your own--whether it's just watching TV, making new friends, or going on your own dates.
Don't feel guilty! It sounds like they're trying to be really considerate of you, but they also sound...goofy and self-involved.
Single, straight, female, solo, non-monogamous.