What is done cannot be undone. What is done happened in the past and the past cannot be undone. By looking to the past, we are perpetuating our mental pain. It’s perfectly all right to learn from the past but it is certainly not so to be obsessed by events in the past. We continue to be a helpless victim if we allow something that took place in the past to be the main thought in our mind, causing other things to be forgotten. If you believe in living your life to the full, it is the now that is important; the past doesn’t matter any more.
It is acceptable to go back to the past and enjoy those happy moments. Some tunes make you remember past events. They really bring back vivid memories of, say, an evening you spent strolling along a beach with a friend who was very dear to you. Having a feeling of pleasure however should not at the same time be to the extent of causing you to feel a slight sadness or arousing your feelings in an exaggerated way about things that happened in the past.