You guys sound a lot like my wife and I when we transitioned from swinging to being poly. We had a very similar conversation where I told her the same thing. I wasn't expecting this to happen!! She had much the same reaction as you did. There's only one thing that you can do in a situation like this. (I mentioned it earlier) You all need to be able to talk openly. I know that you don't want to upset your husband, but these conversations aren't always easy. But they're so important and absolutely necessary to make this lifestyle work.
That brings me to my next question. You said that you like the other woman, but you don't know if you'd have picked her for a partner. Do you think that has anything to do with some of your jealousy? Is there something about her that just resonates as bad or wrong with you? I've learned to listen to my wife's instincts. (I'm a man, so I don't always listen with ther right...tools) Again, these are things that YOU AND YOUR HUSBAND only need to discuss. (Yes, there are times when you and the hubs are going to need some healthy alone time and that's OK!!)
Quick story. My wife liked our current gf, but didn't love her at the beginning of our relationship. It was only after some time that both girls realized they loved each other. I'm not sure if you're bisexual, but that obviously helps that particular situation.
Last and I'll shut up for tonight. LOL If you want this relationship to work, and I mean TRULY want it, then you're going to have to forget everything you ever knew or were taught regarding love and relationships. I say that because you have no idea what the human heart is capable of. (One more story. My wives and I were completely content living as a triad and then BAM, in comes someone that blew all of our worlds away. I didn't think it was possible to fall in love AGAIN, but it seems that's what's in store for us) Use this forum to bounce ideas off of people and to get advice. I wish this place was around when we were starting!!