I think that you are going through the general new-to-poly jealousy and focusing those icky feelings that are being brought up on his cheating. Did you actually forgive him for cheating? It doesn't sound like it. It sounds like that hurt is still there. And yet you decided to go into poly anyway - which would be dredging up a lot of similar feelings whether he was with this woman or someone else.
I think that you need to work on yourself to deal with your feelings, rather than view the problem as an external one. I don't know if I can explain how to work through jealousy well but there is a wealth of resources out there on that. The Ethical Slut is one, Franklin Veaux's site
is another. Here
are some more. Polyamory conferences always have workshops on this as well - there's a list of conferences throughout the US here
. You may also want to find a local polyamory group
to talk to other people about your experiences.
I don't want to totally exclude the possibility that the problem is this woman. If she knew that your husband was cheating, then she might not really understand polyamory. We don't know if she is concerned about your feelings or not. It is important that she has that concern and that she shows respect to you (likewise it is important that your boyfriend do the same for your husband). But simply accepting an invitation that *you extended* is not disrespect. If you are too emotional to understand that then you may not be in a position to judge whether she is being respectful of you or not.
I strongly recommend that do two things: start working on yourself. And try to develop a friendly relationship with this woman. It's a lot easier to villainize your metamour when you don't even know her. Listening to the podcast Poly Weekly
will give you perspective on what it's like from your metamour's perspective, especially episode 330. Dating someone who is in an established marriage is not the easiest thing when you have the best of intentions.
If the podcast format works well for you, I also highly recommend these episodes of Poly Weekly: 311, 302, 293, 298, 277, 276, and 274.
Poly is hard work! When things are difficult, remind yourself that it's worth it - you get to have this awesome new experience with your boyfriend, and you don't have to sacrifice your relationship with your husband for that. That is the reason for all the hard work.