Assalamu Alaykum I am addicted to a sin. Every time I do it, I repent bi idhnillah, but the effects of the sin (qaswatul qalb, sadness, low iman) remain on my heart. What is the best way to soften my heart, regain my iman, and undo the effects of the sin given that I have already repented from it.

Keep repenting and cut off whatever keeps taking you back to it, the environment, the friends, the phone, because as long as the way back is open the heart does not settle. Imam Ibn Baz named the cure for hardness of the heart as reciting the Qurʾān much and with reflection, much dhikr, much prayer, lawful earnings and sitting with the righteous, so busy yourself with these and the heart softens in shā' Allah.