The past is the past. Sometimes it is hard not to go back and relive it. However, God is telling us to do just that. We are not have lives of regret, but of hope and expectation of things to come.
If we have made mistakes in the past, they cannot be undone. But they can be brought under the blood of Jesus so that the mire of disappointment and self-condemnation can be cleansed and made pure.
Today, don't dwell on the past. Look to the One who knows your past, present and future. Even with that knowledge, His love for you is fierce and unwavering.