Forget the former things; do not dwell on the past. Isaiah 43:18
How successful do you think you would be if you drove your car always looking in the rear view mirror? The truth is - you wouldn't get very far before you crashed! Why then would we think we could be successful in life if we continually dwell on our past failures?
The gospel of grace communicates a clear message of God's free gift of righteousness (Romans 5:17). A righteousness that is not attained by our behaviour - good or bad! On the cross, Jesus took all of our sin and gave us all of His righteousness (2 Corinthians 5:21). As far as the east is from the west so has He removed our sin from us (Psalm 103:12). The old has gone, the new has come (2 Corinthians 5:17).
Paul knew that looking back affects forward movement, that's why he said that to effectively reach forward to the things that are ahead, he had to forget those things that were behind (Philippians 3:13).
Don't let your past determine your future.