I've always wondered about the usage of the word "forget" in the scriptures with relation to receiveing answers to prayers and repentance. Elder Gerald N. Lund in his book, Hearing the Voice of the Lord, made the point very clear with regards to receiving answers to prayers; and I wondered how the same thing could apply to forgetting one's sins after true repentance.
I don't think it's possible to literally forget that we ever did anything wrong no matter how well we repent. Part of not forgetting our sins probably helps us to remember the pain that we felt upon making certain wrong decisions. I was thinking that I needed a better understanding of the word "forget." Elder Lund talks about the part in Doctrine and Covenants 9 where the Lord tells Oliver Cowdry how to receive an answer to a prayer. He says that if we're not praying for the right thing or the answer is no, "you shall have a stupor of thought that shall cause you to forget the thing which is wrong." This doesn't mean that we will literally wonder what in the world our "wrong" choice was, but that our desire to follow that choice will no longer be a strong desire. The same holds true with repentance, by "forgetting" our sins, we're basically forgetting "our desire" to commit the same sin, which is what true repentance is.