Wow, I'm in a very similiar boat to you, inlovewith2.
My partner is monogamous (and happy/proud of it) and I have only come to realise during the time I have been dating him that I *am* poly, not that it's a lifestyle choice which might work for me.
I still can't explain about the reason why I desire more than one lover at a time. There was a counsellor I saw once who said something like: "We could explore why you feel you need it, what you get from it." And the only answer I can think of for that would be "love".
You can never have too much love, right? I know you can certainly have too little. A group can achieve much more than a single pair.
I'm holding back on taking other partners but it does feel like trying to "convert back to being mono" for me. If it were just painful then I think I would do it, I really love this guy, but the problem I really face is that I'm not sure if I can and I'm not sure how to figure out if I can without long periods of suffering on both our parts.
I hope you guys work it out, sorry I couldn't give any advice on it, and thanks for showing me that there is someone out there who knows what I'm going through to some extent!