Life is a little bit like racquetball. To look back at your past mistakes and relive your failures will only cause damage. To dwell on the past can ruin your relationships, lower your self-esteem, and knock the confidence right out of you. To live in the "I wish I had done it different" or "Why did this happen?" will smack you in the face and cause you to have nothing but distress.
You cannot move forward if you're looking backward. You cannot undo your past, but you can do something about your present and future. You can only play the ball that is in front of you, not a ball that's in back of you. Do what the apostle Paul did: "Don't look back!" The Bible says that God will take what the enemy meant for evil in your past and He'll turn it around and use it to your advantage. God has a plan for your past, but He can't work in your past because He works in the now!