I see many of the advice here approaching the situation from a swingers perspective (which I know very well also) rather than that of a polyamorous perspective. So here is my take on it.
Jealousy, as you know, is insecurity. It's the fear that someone else may be "better" or valued or desired more than yourself by your loved one: your husband. Sure, there are things that can be done to reduce the jealousy a little bit, but nothing short of fixing yourself will eliminate what you are feeling. And by "fixing" I mean understanding that you are worthy, you are "enough", you are awesome and you do mean the world to your husband.
That said, if he is in love with her just ending it like many have suggested won't work - it's neither fair to her, nor to your husband who could end-up resenting you for the implied ultimatum.
In my experience vetoes can work in swinging, but they don't work so well in polyamory. With the emotions in the game the stakes are much higher when trying to control your partner's actions and who they are in a relationship with besides yourself.
On top of that, his girlfriend is not just a shoe that can be thrown away, she also has a vested emotional interest in your husband and you.
My advice, and it's not easy, is to look deep within yourself and figure-out what it is that is triggering such strong emotions of jealousy. Then, when you think you can articulate it well, sit down with your husband and the other woman and talk it out. Let both of them know how you feel and why you are feeling it.
They can't fix your feelings of jealousy, but they can empathize and maybe help you work through them. I think you'll get further working on it as a triad than just as a couple.