Hi. Im the husband in a mono/poly relationship. We are in a V with her as the hinge between her new man and me. I know the emotions you are going through. I am in a bit of a different situation as I am the one who suggested she go poly while assuring her that I would stay mono because of her jealousy issues.
As it was a fantasy of mine for years before we actually took the plunge, I did imagine all th epossibilities before deciding that I was ready. It doent make the emotions any easier. They are meeting tomorrow alone for the first time and I have this emotion fo intense excitement coupled with that nagging fear that she will lose interest in me, leave me, etc...and yet I know that if I give in to the fear then I WILL fulfill my own fear-she wont be interested in me if I am whiny and clingy after suggesting to her to date him.
Your situation sounds a bit different like he wanted to be poly and convinced you to let him. Are you not interested in being poly too? Or is it because he does not want you to be?