Is the absence of feeling remorse for some sins when repenting, and merely desisting from them and resolving not to return to them, sufficient for the acceptance of repentance, or is feeling remorse a condition for that?

The path to achieving remorse is outlined in Fatwa No. 107223. The decision to repent and cease sinning, as well as the fear of Allah's wrath, must stem from remorse over committing the sin. Every repentant person must be regretful, and Satan creates delusions to discourage repentance. When the harm and danger of a sin to a servant's religion and worldly life are understood, remorse results. See Fatwa No. 172105.

Therefore, one must exaggerate in contemplating the gravity and harm of the sin, and the intensity of Allah Almighty's wrath upon sinners, so that remorse increases and the pain magnifies. For further benefit, refer to Ibn al-Qayyim's discussion on preferring to forget or remember a sin in Fatwa No. 119062.