Because of this letter, it is quite clear that not only did Sauron carry the Ring while he was in Númenor, it was actually the Ring's power that allowed him to so quickly corrupt the kingdom.
In the Second Age, Sauron frequently disguised in "a fair form", and took the false identity of Annatar, "Lord of gifts", among the Elves and in the kingdom of Númenor, which he intended & succeeded to destroy from within.
Sauron was finally defeated in SA 3440 in the War of the Last Alliance, with Isildur cutting the One Ring from Sauron's hand. But he was not vanquished for his Ring was not destroyed. Instead, it corrupted the King of Gondor, and thereafter the Ring was known as Isildur's Bane.
The Númenóreans and their ancient island realm are referenced in passing during The Lord of the Rings, but don't physically appear (for good reason - more on that later).
Was the One Ring forged before or after the fall of Numenor?
The Rings were forged long before the fall of Numenor. Once Sauron conquered Eregion and seized the 16 he'd helped make, he gave 9 rings to evil men in order to control them.
Although Sauron's body was destroyed in the Fall of Númenor, his spirit was able to bear the Ring back to Middle-earth and he wielded it in his renewed war against the Last Alliance of Elves and Men between SA 3429 and 3441.
Sauron made the One Ring for himself alone. He never expected to lose it, either. Sauron came in a pleasant disguise as Anatar, Lord of Gifts, and taught Celebrimbor and other elves the art of making magic rings. In addition to the One Ring Sauron made for himself (and made last) there were 19 other Rings of Power.
After lying hidden and increasing his power in secret, in S.A. 1200 Sauron put on a fair visage, calling himself Annatar, the Lord of Gifts, an emissary from the Valar.
The Númenóreans brought him back as a prisoner. Through Sauron's cunning and persuasion he rose from prisoner to adviser for the King, convincing many officials to worship Morgoth, they quickly became corrupted to his side.
I think it's probably fair to say that the Dunadain of both Arnor and Gondor considered themselves Numenoreans in exile. Aragorn himself told Arwen on his deathbed that he was “the last Numenorean”.
Sauron seemed to genuinely feel a connection to her, and part of that came from sensing the darkness inside of Galadriel. In that regard, his offer was really a seduction to the dark side.
There could have also been a darker reason that Galadriel didn't want to confess the deception. The events of the Lord of the Rings proved that Galadriel wasn't above temptation, further reinforced by The Rings of Power.
The Elvish writing on the New Ring translates to "I renounce the Blessed Realm. To redeem the Land of Shadow. And bind the walls of Arda, In place of the Dark Lord, You shall have light undiminished, All shall fear me and rejoice."
Sauron did not fear as we think of the word. He was very cautious of the men of Numenor and the remaining Noldor in exile. No other force seemed to give him pause since the First Age. This was his undoing.
In The Lord of the Rings: Return of the King, Éowyn asks Aragorn his age and is shocked to learn that he is actually 87 despite looking no older than 40. But it is revealed that Aragorn is one of the Dúnedain—a descendant of Númenor—blessed with long life.
Sauron's forces were driven back from the Grey Havens, and retreating to the Gwathló found their way blocked by the second Númenórean army. Sauron's army was thus pinned between the two forces and mostly wiped out in the ensuing Battle of the Gwathló.
For The Rings of Power, Númenórian hatred of Elves built over time out of jealously for their immortality. In The Silmarillion, Tolkien outlines the downfall of Númenor as a slow burn that began with the Ban of the Valar, which entailed the first Númenórians agreeing to never sail west toward Valinor.
The Númenóreans made Umbar, the harbour city in the south of Middle-earth, into a great fortress and expanded Pelargir, a landing in Gondor near the Mouths of the Anduin. The "King's Men" among the Númenóreans became jealous of Elves for their immortality, resenting the Ban of the Valar, and sought everlasting life.
Morgoth is the more powerful of the two. Morgoth was the mightiest of the Valar, a group whose weakest member would outpace Sauron. However, there are some circumstances and eras in which Sauron would win the day. Morgoth was born far beyond Sauron but lost and surrendered much of his power.
Saving Galadriel at sea was likely a method of gaining her trust, since Sauron would need an Elf's friendship to enter their realm. Whether he targeted Galadriel specifically because he could exploit her bloody-minded rage and broken heart is unclear.
Bloodlines are very important in Tolkien's stories, and it makes a lot of sense that Sauron would fear the great-grandson of the man who cut off his hand. Aragorn carried the blood of Numenor with him. His genetic connection to the heroes of the Second Age established him as a dangerous opponent of Sauron.
The Three Rings were magical artefacts, three of the Rings of Power. They were forged by Celebrimbor and granted the power to preserve the beauty of things by stopping change and warding off the decay of time and postponing the weariness of the world.
The Second Age ended, and Isildur became High King of both Arnor and Gondor. Isildur took up the hilt-shard of Narsil, Elendil's sword, and cut the One Ring from the hand of Sauron.
Sauron (Sala Baker) directly made only one of the 20 rings, the famed "one ring to rule them all," although he assisted in the creation of the nine rings for mortal men and the seven rings for the dwarves. The three rings for the elven-kings were forged alone by Celebrimbor, with knowledge obtained from Sauron.
