A short memory is said to be good in basketball. If you miss a few shots, a basketball player is encouraged to have a short memory so that they will not give up on shooting the ball. Each shot is a new shot. In the area of sin, we need to have a short memory in that we need to put behind what is past and move toward where God would have us. Seek forgiveness and then move forward.
Philippians 3:13-14 (ESV)
13 Brothers, I do not consider that I have made it my own. But one thing I do: forgetting what lies behind and straining forward to what lies ahead,
14 I press on toward the goal for the prize of the upward call of God in Christ Jesus.