I think that we are going to need a lot more specific information before we can give you much more specific advice. What was the state of your marriage when you tried polyamoury before? What did you do to try it? What happened that it did not go well? More recently, we need to know some things that, from the sound of it, you need to find out yourself.
As several of the other members have mentioned, polyamoury can mean a lot of different things. I would not rely on such broad terms in any of your discussions while you are at such a juncture in your life. Trust me, it pays to be specific.
You need to know exactly what your fiance needs from your relationship. Does she want other sexual outlets with no emotional connections? Emotional connections but not necessarily sexual ones? Does she want to add a partner to your relationship together to form a triad or does she want relationships separate from yours? Is she interested in males or females?
You also need to know what you need from your relationship, and be able to explain it to your partner. What exactly does her suggestion make you feel? What about the suggestion makes you feel that way? What are the requirements that you have to be in a relationship with a person? What are your boundaries? What do you need to have security and trust? What do you need to feel loved and appreciated?
It sounds like you have a lot of talking to do with your partner. I think reaching out to the poly community was a good step. I hope you are able to find balance and happiness in your relationship.