Is forgetting an ongoing sin considered disbelief, even if it is not belittled, knowing that belittling a sin is disbelief?

The statement that belittling a sin is an act of disbelief (kufr) is false. It only becomes disbelief if the belittling is directed towards the One who forbade it, which is Allah Almighty, and involves disregarding His right and being indifferent to His command. These are matters of the heart that manifest when the sinner explicitly states that they do not care whether the act is forbidden or permissible, and that they are indifferent to Allah's pleasure or displeasure. There is no necessary correlation between belittling a sin and disbelief, nor between persisting in it and disbelief. The examples mentioned do not render the sinner a disbeliever, and one must be wary of over-elaboration, rigidity, and obsessive doubts in matters of disbelief.