Just because you are the maker of something, doesn't mean you know everything about it/how the thing you make will turn out.
I can attempt to make my sister's raspberry coffee cake for the 1st time, and I doubt it will turn out just the same, or just as good, even if I attempt to stick strictly with her "recipe."
When Saruman made his own ring, I doubt he was wanted to have a piece of jewelry for himself, he was trying to mimic Sauron and make his own Ring of power. As far as we know, it failed, and I doubt that was the outcome Saruman was hoping for...
So, simply because Sauron created a powerful Ring, that does not mean he knows everything about it. Sauron had the ability to reform, and re-shape into bodies before creating the Ring, why would his ability to do so after losing it, make him believe the Ring wasn't destroyed?
For as much as we know that Sauron believed no one had the will power to destroy his Ring, people will want to use it against him, maybe he only realized that when he was defeated by Isildur? First he assumes...well guess it was destroyed, too bad. Then after learning that neither the mighty Isildur, nor the stuck-up Elves, wanted to destroy it...that must means none of these weak humans (of the 3rd Age) will want to either. They will only seek to destroy me by using the Ring.
As far as Sauron recognizing his peril when Frodo reached Sammath Naur, that doesn't mean he knew that 2,000 years ago. What Sauron does figure out, is that he was 100% wrong about people not wanting to destroy the Ring. He always assumed the Ring's only threat would be if it was used against him, well if someone's taking it to the place where it can be destroyed, this proves Sauron was wrong, enter in doubt, concern, and fear that he could be wrong about the Ring in other ways too...not only now does someone want to destroy it, but mayber there is a powerful enough will to destroy it! (I'm only theorizing about Sauron's thoughts above, who can say for certain?)
