Ladyinticate, I can certainly feel for you. My wife just recently discovered she is poly. I'm pretty sure I'm mono, but hope that I can give poly a try as well.
It's been difficult for me. The last 6 weeks have been a very interesting roller coaster ride. From my experience though, it's certainly been positive. I learned just how much I love my wife. She means everything to me. Our love has grown leaps and bounds, which is something I didn't think could happen like that.
I have a hard time dealing with the fear of loss, no matter how irrational it is. All Jen can do is to reassure me that everything is fine, and she's not going anywhere. Thankfully, we have embarked on this journey with a mutual friend, that has a lot of respect for what we have, and for each of us.
It hasn't been an easy transition, but we are all living it day to day, and we feel better for it.