[Galadriel55]

But even Theoden, with, as you said, no particular evil trait, needed someone (Gandalf the mentor) to guide him into the light again. Just like Gollum. And Denethor would have been like that too, had he listened to anyone in his last few hours.

Lobelia, Thorin, and Boromir see the consequences of their deeds when a disaster happens. A disaster they still have a slim hope to fix. Gollum, I would think, never really thought about his position that way. And Denethor refused to see any hope.

Perhaps if Sauron really saw what he and Morgoth did as a disaster, or if he had a mentor who convinced him of such, he would have repented for good. But seeing as that what we consider a disaster Sauron considers a great success...