Is cursing in the heart, without attempting to repel it, considered a result of discontent with Allah's decree—like the obsessive cursing that one repels—thus leading to apostasy?

What comes to mind of the whisperings of insult, without speaking it, acting upon it, or assenting to it, is forgiven. Its owner is not considered an apostate. However, this is dangerous and requires treatment. A Muslim should seek refuge with Allah from Satan when these thoughts arise, and not surrender to them. He should also avoid isolation, occupy himself with beneficial knowledge and inviting to Allah, while frequently contemplating Allah's blessings and pondering the texts of revelation.