Ok, I'm going to suggest that you read the book "The Ethical Slut". This book has done wonders for my wife.
There is a part in there which deals with jealousy. One of the authors said they had a issue with jealousy ones, and they asked their significant other to re-assure them that they (the sig other) wasn't going anywhere, and they would still love them (the author). My wife has done this to me since our marriage began....almost 20 years ago. She did it on a regular basis even though we weren't practicing poly yet. We didn't even KNOW about poly.....She has done this since we've been married. I, on the other hand, have not....So as we waded into this poly life, who do you think had the largest issues with jealousy? Thats right, she did. It's my own fault.....because I was lacking in my reassurance to her that I would not leave, and that I would love her eternally.
Ask her for that reassurance. Tell her that you are scared and/or nervous and most of all, WORRIED that she will leave you for him.