What is the ruling on one who settles in his mind a self-talk (حديث نفس) about Allah Almighty, such that if a person were to utter it, he would disbelieve, knowing that he did not speak it or act upon it? And does he disbelieve by that?

It appears the questioner is afflicted with waswasah (obsessive whispering/doubts), and what she mentions is a deception from Satan and an illusion to cause her distress. The advice is to ignore these waswasah and not to pay them any mind, and to know that she is on the right path as long as she strives against them and seeks to be rid of them.