Does repentance from sins require repenting from each sin individually, or is it permissible to repent from all of them at once? What about forgotten sins? And what is the ruling on fearing a lack of sincerity in repentance due to the possibility of committing the sin in the future?

It is permissible to repent from all sins with a comprehensive repentance, even if a person has forgotten some of them. So, whoever repents from all his sins, regrets his shortcomings, and resolves not to return to them, his repentance is sound and accepted. Abstaining from repentance for fear of returning to sin is a deception from Satan, so one must hasten to repent and be truthful with God. If the believer returns to sin, let him return to repentance, for the believer was created "prone to trial, oft-repenting, forgetful; if reminded, he remembers."