“One thing I do: forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus.” (Philippians 3:13-14)
The life of a Christian is not focused on the past, but on the future. Our goal is to be fully united with Christ, forever.
A runner who is focused on his race has no time to cast his eye over his shoulder to mark the steps already trodden. He concentrates on the goal that lies before him. Similarly, Christians should not focus on their past failures in so far as these might limit their confidence in the certainties and victories of the future.
And on the other hand, they should not remember their past achievements when these tend to make them proud and self-confident. The apostle Paul is a good example here. He had a lot to be proud of: his origins, his religious education, his blameless obedience of God’s law… But no, he says, “I count everything as loss because of the surpassing worth of knowing Christ Jesus my Lord.” All his own achievements could not contribute to his salvation. They were not worth remembering.
Of course, we need not forget God’s grace in the past, or how He has saved us from particular sins and weaknesses. But we should not linger so much in the past that we forget about our life goal: to be pure and blameless for the day of Christ (Philippians 1:10).