I would say that the Fizik Anatres is more similarly shaped to the Prologo Nago, although the Antares has less padding than the Nago. The Arione has more padding than the Nago. But none of these have a cut-out so they will definitely feel different if you're used to a saddle with one. All of the saddles mentioned are quite flat.

Maybe you're having difficulties with the new saddle because, well....it's new. You did mention that your old saddle was saggy and bouncy and your body has become accustomed to that.