Actually, I think that you're handling this rather well. I'd just like to throw in a few ideas that may or may not be pertinent.
1) Jealousy is something that was sold to most of us as a necessary ingredient of true love. Don't expect to get rid of it overnight.
2) You write that your husband is a member here. Has he read this thread? Have you talked about the comments you're getting here - as well as what you're expressing yourself? Is she on here as well?
3) I'm a big fan of the two more distant members of a V (or in your case a triad) making an effort to get to know and develop their relationship independently of the middle-man (or -woman). How would you feel about a "girls' night out" with her? (Just for starters.)
4) You seem to have dealt with this point positively with your husband, but they both need to be clear that they both cheated on you and they both have to work on regaining/winning your trust. This is not a matter of punishment or paying for their past crimes, but of repairing bridges. Anybody who prefers to drive over a shaky bridge rather than repairing it is asking for trouble.
5) Having said that, I see signs of your husband's maturing. One is accepting his responsibility for the hurt that he's caused you. Another is that instead of asking you to become like the women that he finds (momentarily?) attractive (criticising you), he is accepting that he can't get everything he wants from you, but can love you as you are and get some variety from the other woman. (The serial-monogamous solution was "throw the old banger away and get a new model... then another one... then another one...") Another is agreeing to proceed at your pace.
6) TruckerPete has already mentioned this, but I want to agree: that although you're the one who's new to their relationship, she's the one who's new to yours and his. This isn't one of those cases where one person holds all the aces in their hand. As long as all 3 of you are willing to work on it... to be willing to want to trust each other and build on that hope for a healthier relationship, I think that you stand a fairly good chance. It might - as RedPepper points out - develop into a V. But isn't even that an improvement on being traded in for the new model... or continuing to be cheated on?
If I can't dance, I want no part in your Revolution.
- Emma Goldman Anarchist and Polyamorous par excellenceThe person who says something is impossible should not interrupt the person who is doing it.
- old Chinese proverbAnd the day came when the risk to remain tight in a bud was more painful than the risk it took to blossom.
~ Anais NinI'd rather have a broken heart / Than have a heart of stone.