Heretic’s Fork: Is there something your muse will never confess to?
Melkor is… very candid in regards to his feelings on things. Part of this may be just a villain’s need to soliloquize and part of it is just that he doesn’t feel a lot of shame. XD
So the things he wouldn’t confess to I think would be mostly things that he would also never admit to himself; he will never confess to his doubts about his rebellion (he doesn’t have many, but he also receives no guidance from anyone, and spends a lot of time alone in his own mind. Of course he has doubts sometimes— but he also has the conviction that he was right.)
The biggest thing is that he will never, ever confess or admit that the Silmarils are his greatest weakness, or that his dependency on them is a leading cause of his deteriorating health and sanity. He will also never admit that he needs help— particularly, he will never admit that he needs help from his siblings who are all lost over bridges he’s already burned.
Sauron for similar reasons will never admit (in later ages) that he is stuck in a downward spiral, that his very need for control is born out of desperation and panic, or that his compulsion to micro-manage every aspect of his armies and dominion spreads his power so thin that it makes him vulnerable.
Basically, neither dark lord is in a good position to admit to the foibles that will cause their own downfall, and even if they are aware of what is happening, they are unable to change course.
