Thanks Trucker Pete and Rob Fire for the thoughts.
TP--you're right that she's the one coming in late--my husband and I have been together for 8 years, married for 5. I think that in this situation, it's hard for me to not think of it as me being brought into their relationship though, because they are in love and I'm looking at trying to develop my own relationship with her. Although I guess I could also look at it that WE are in love and she's trying to develop a relationship with me. Because of the sneaking around thing, where she knew about me and I didn't know what was going on with her, I think it makes it feel more like I'm the intruder at times, though.
Rob--the feelings of jealousy no doubt are somewhat related to the usual feelings of inadequacy and fear that I think come with anyone embarking on polyamory for the first time, especially if it wasn't something that they sought for themselves. It's hard to shake a lifetime of conditioning that says there's something wrong with you if your partner wants someone else. Not to mention the fact that over the years when my husband has had feelings for other women, it has often turned into discussions about why can't I be more like they are. He has not been doing that here and has apologized for doing that in the past, but, again, it's hard sometimes to shake the past.
I agree that we do need to work on the betrayal of trust and have been doing that. He gets frustrated at times with my time-table and wonders when I'm going to get over my feelings, but I do feel like we're in early days here and that in all fairness it's a lot to expect that I would be over everything in 4 weeks.