[Belegorn]

Zigûr is right. The Númenóreans were already divided into two Parties before Sauron came. Those who were friendly with the Elves & revered the Valar, and those who were not and both Parties envied the Elve's Doom. This crystallized during the reign of the 14th King of Númenor.

"the greater party... called the King's Men, and they grew proud and were estranged from the Eldar and the Valar... the lesser party... called the Elendili, the Elf-friends; for though they remained loyal indeed to the King and the House of Elros, they wished to keep the friendship of the Eldar, and they hearkened to the counsel of the Lords of the West...they, ... did not wholly escape from the affliction of their people, and they were troubled by the thought of death." [Sil, Akallabêth, p. 328]

This ate at them badly. They really wanted to be as Elves and regretted their forefather's decision to be a Man. Sauron only played on this motivation of the Númenóreans and used it to aid in their downfall. It was really the only way he could beat them. He knew his armies stood no chance against them and when he saw the armies of the High-Men he realized they were even greater than the tales of their majesty. Imagine that! So Sauron did what he did best, he worked his magic in ways to get them to destroy themselves.

There was an attempt to get things to rights by the 2nd to last King of Númenor but the damage was already done and most of the people did not agree with the King anyway and repentance was not accepted by the Valar. He gave the Faithful a blow while he was around but otherwise they were always persecuted by the Kings Men and forbidden to engage with Elves or to speak their language.

They did themselves in and Sauron helped to speed it up.