I pray you never will know it. I don't think I've ever felt anything else like it, like I was being eaten up from the inside, sucker punched, betrayed at the most visceral level.
I have never been unfaithful, never cheated on a boyfriend, or my husband, even when my marriage was at its absolute worst. The sad thing about his cheating is that it began early in our marriage, long before it became a difficult relationship, so there wasn't even the excuse that things were awful. He just did it because he could. I still kick myself sometimes because I was so completely clueless for the longest time.
Personally I've found nothing positive in feeling jealous or envious. I have always experienced them as very negative emotions, not motivating at all, instead just causing depression, apathy, and an overall misery. For me, they are not the kind of emotions that get me moving, get me motivated, that spur me on to action. Instead they just make me want to climb back into bed and pull the covers over my head in despair.
My fiancé IS an attractive and appealing person.

I've seen other women eyeing him appreciatively.

Makes me proud to be with him. I do nice things for him because I care about him, not from any sense of competition with anyone else for his time or attention.