This is a devotional summary of Elder Holland's talk Remember Lot's Wife.
Elder Holland said to leave your past sins and mistakes there, and don't keep digging it up. Don't keep digging up your spouses past sins and weaknesses either, because that will result in him/her digging up your past and slinging mud in your face.
Being unwilling to forgive is mockery of the Lord's atonement, because it sends the message that He can’t soften and heal hearts. The greatest miracles of Christ are His ability to soften and change hearts. He can change a hard, bitter, hostile and angry heart, to a soft, loving, sensitive and forgiving heart. That is one of the Lord’s great and marvelous works.
Refusing to forgive yourself or others is the worst kind of wallowing in the past.