Letting go of the past does not mean forgetting the past - it means letting go of the emotional baggage that we have accumulated in our many experiences. It seems like our minds have been conditioned to think and act in a certain way, hanging on to everything emotion (especially unpleasant ones) and we bring all of it into our lives everyday. It is
as if we recreate the past in the present moment.
For instance, if we had a relationship several years ago that ended sadly and we still feel the pain when that person comes to mind, maybe great pain, we can ask ourselves why we feel so much emotion over something that does not exist anymore. How can we feel so bad today over something is totally gone? I think the reason is that we are recreating the past in the present. Its not what happened in the past that is the source of the problem, its what's happening in the present. Dragging the past into the present and reliving it as if it were real is a very bad habit. It will flavor our minds with sadness and it will make it very hard to do anything truly new, truly fresh and it will cast a shadow over our joy.
I have heard people say, and I have said it myself, that when you suffer a painful loss you never get over it completely. Today I find this idea horrifying. If it is true that you never completely get over losses, then life will always be filled with pain piled upon pain. By the end of our lives, we will be very, very sad.
Buddha's teachings on impermanence and subtle impermanence contain a powerful method to counteract the habit of hanging on to past pain. Habits can be changed - we can replace pain from the past with a day by day happiness in each moment. Again, easier said than done. I have all of the societal thoughts and training most Americans grow up believing about loss and pain. Its takes concerted effort and a lot of time to change these habits. But its so worth it.