What can I do to genuinely regret a grave sin I have committed when I find no true remorse within myself? And is delaying repentance a sin that necessitates repentance?

Repentance from sin is immediately obligatory by scholarly consensus, and delaying it is another sin. For remorse to be achieved, which is one of the conditions of repentance, one must remember the Creator's greatness and His punishment, and that disobedience may be a cause of affliction and destruction, and it will be recorded in the book of deeds on the Day of Judgment. Allah Almighty says: (And return [in repentance] to your Lord and submit to Him before the punishment comes upon you; then you will not be helped) [Az-Zumar: 54].