Hello Who Am I,
Welcome to our forum.
I think you are kind of in the mourning process of having thought you and your fiancÚ were "The Only One" for each other. Give yourself some time to just get used to this radical change in things -- and I'd even suggest put any wedding plans on hold until you get through this. You may have to decide whether you want to stay together. Is a poly relationship something you can live with? Do you feel like it's worth it?
GalaGirl and polyhope both had good things to say too. Give those posts some thought.
Glad you could be with us,
Love means never having to say, "Put down that meat cleaver!"