And thus did the Spirit of the Lord work upon them, for they were the very vilest of sinners. And the Lord saw fit in his infinite mercy to spare them; nevertheless they suffered much anguish of soul because of their iniquities, suffering much and fearing that they should be cast off forever. (Mosiah 28:4)
It is true that the Savior has paid the full price for our sins, but that does not mean that repentance is painless. Repentance means change, and change can be excruciating. We know from D&C 19:16-17 that we must suffer to pay the price for our own sins if we do not repent and let the Savior pay the price. But repenting itself requires some level of suffering, as Elder Dallin H. Oaks explained:
“The repenting sinner must suffer for his sins, but this suffering has a different purpose than punishment or payment. Its purpose is change” (The Lord’s Way [Salt Lake City: Deseret Book, 1991], 223