Any motivational speaker, counsellor or self-help guru worth their salt will tell you straight away to get away from your past and live in the present for the future. They will also remind you that hanging on to things that happened in your past is detrimental for any future growth, and in fact, will hamper any attempt to move ahead. Of course they are right. I believe that this valuable piece of advice is straight from the heart of God - He, after all, was the first one to come up with the idea that asking forgiveness for a mistake can result in the total eradication of the mistake! The really hard thing for most of us to come to terms with is not God's forgiveness, but rather the ability to forgive ourselves. So yes, we all need to move away from the past and focus on the future.
However, there is an instance where it is good to remember the past. Please note that I am speaking about a simple recall and not some morbid, introspective digging-up-old-bones! The reason I believe that our past is sometimes good to remember is that it will keep us humble in our present and pave the way for a successful future. Paul writes to the Corinthian congregation, "My brothers, remember what you were when God chose you" (1 Corinthians 1 :26). He encourages them in the context of his conversation showing how the power of God can transform our very nature. Paul says that before encountering the gracious power of God, "not many of you were wise by the way people look at it. Not many of you had power. Not many of you came from a family with a big name.But God chose things that look foolish to the people of the world. He has used those foolish things to put the wise people to shame. God chose the weak things to put to shame the strong people" (1 Corinthians 1:26-28).
So, by remembering our past we understand what we were before our God encounter! In the proper context this remembrance can serve not only to keep us humble, but also empower us to help other people with similar past experiences. Paul has the same advice for the believers at Ephesus by reminding them what they used to be before coming to know Christ. It is good to remember where you came from since it cultivates an appreciation of what has been done for you. In remembering our past in this manner, God is glorified, and therefore it is good to dwell on the past.
Our past sins and mistakes also works as an early warning system when faced with similar temptations and issues. The old English adage says it well - once bitten twice shy. So there are times when it is good to dwell on the past, albeit briefly, in order that we can live our present and plan our future.