Hi and welcome
I don't think this is a lol scenario at all. I think you have made a promise that you are now doubting you will be able to keep, and you are probably right.
I started a thread called "the downsides of being poly" and this is one of them. Monos and polys falling in love is a difficult thing but definitely one you need to address before you get married.
My poly partner tried to give up a a significant other for me and in the process I lost so much of him that I told him I would come to terms with his polyamory. The easy thing for you to do is to tell yourself and your fiance you have this 'handled' and you may have it handled this time, or for a while, but it will come back to haunt you.