Natalie I don't have much advice, but you're definitely not alone. I'm pretty new to poly and the girlfriend (for lack of a better word) of a married couple (we're a triad, all together). I freak out A LOT, I wonder if I can handle it, everything you're talking about. But I do know that I love being around my couple (they're away from me right now and I'm miserable!) and that makes me want to continue, despite all the worries. I definitely don't think that being worried about it means it's not for you. If you're willing to work at the things that bug you, and you decide you want it, then I think this can work for you. I'd just say to be honest about it and hopefully your man (and hopefully, his wife) will be supportive. Try not to freak out and run off
I've done it a few times, it helps notadamnthing!!!