True repentance requires that we surrender the memory of the sin, not in the sense that we are without the knowledge that. we once transgressed but rather in the sense that we have laid down the burden, that our confidence might now wax strong in the presence of the Lord (see 2 Nephi 10:20). If Alma was without any memory whatsoever of his sin, he could not have given this recitation to Helaman.
We must retain sufficient memory of the pain to avoid a repetition of the suffering. Still, as we grow in the things of the Spirit, that which is forgiven is to be forgotten. It is not true repentance when we cling to a sensuous memory in whose mental replaying we find delight. “Behold, he who has repented of his sins, the same is forgiven, and I, the Lord, remember them no more. By this ye nay know if a man repenteth of his sins-behold, he will confess them and forsake them.” (D&C 58:42-43.)