Why Is PHYLLIS So Desperate ...?

That's exactly my point, that is because she "filched" Nick away from Sharon, and that was where his "heart" belonged. With her is just infatuation, they are just sensual soulmates, and that is what they have mistaken for love. Phyllis knows, and can see that there is a connection between Nick and Sharon that will NEVER cease no matter what because they were solid as a rock, and their marriage was built on shaky ground, so it was rocky from the start.