Hi Tech - I am in your same situation. I'm mono, last year my husband fell in love with a co-worker (who is also married) and discovered polyamory. I don't really have any great advice because I struggle with the same issues you do. You can check out my blog (KT's Blog) and read about the ups and downs that I've been through. MANY people offered me great advice that you could apply to your own situation. There are other great blogs and threads that you can read where you can find support, advice and understanding.
I struggle with this everyday - I think it's ingrained in us that this is wrong, that there must be something wrong with us or wrong with our marriage if our spouse falls in love with someone else. We had been light swinging for a few years before he fell in love - but when he was with other women, I was never jealous or insecure. Sex was just sex. It was fun, a thrill. I got incredibly turned on by watching my husband with other women. With his girlfriend - it is different because it's not just about sex. It's that deep intimacy that they are sharing that before he only shared with me. As time goes on, it is getting easier and easier but there are days when have a hard time. Those times don't last long - the trick is learning how to ride them out. I'm still learning how to do that.
My advice is to read, research, communicate. It's a learning process. It will help if you understand polyamory a little better and if you are willing to really take a deep look at yourself to dig up your insecurities so you and your wife can talk about them. I have never had to do so much self-exploration and self-analyzing than I have this past year. A relationship therapist might be a good person to help you do that.
Best of luck!