How can one be guided by God to repentance and remorse for disobedience after backsliding and relapsing into sins, to the extent that the heart is no longer affected or fears God, and has come to love sins and not regret them?

Do not despair of the mercy of Allah, for the door of repentance is open. No matter how great the sin, one must abandon it, regret having committed it, and resolve never to return to it. To soften the heart and incline towards repentance, reflect on Allah's Names, Attributes, and Power, and remember death and the Fire. Strive in supplication, and accompany the righteous.