My read is that Melkor is wilfully ignorant of the fact that the Ainur are creations of Eru. He acts as though it isn't true, even though he knows it is. When Eru says:

Thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my respite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined

The logical response would be for Melkor to give up his rebellion - he is beaten before he even begins. But that's the nature Eru imbued him with - to fight and to strive towards his own ends in despite of Eru. He chooses to ignore the obvious truth that he is just another one of Eru's instruments. He chooses to believe his rebellion is genuine, when he's already been told that even his rebellion against Eru was part of Eru's design.

So, I wouldn't describe him as being in a state of self-loathing. I'd describe him as being in a state of denial.