How can I guarantee Allah's concealment of my sins in this world and the Hereafter after repenting and abandoning the sin?

Sincere repentance erases what came before it. Allah accepts His servant's repentance and conceals their sins in this world and the Hereafter. Rather, He transforms their evil deeds into good deeds, as Allah the Almighty says: "Except for those who repent, believe and do righteous deeds. For them Allah will replace their evil deeds with good deeds," and as the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin."