In Ancient Indian Scriptures, they say, all your thoughts come from the past. if you develop a skill not to give attention to the past, the mind will be free of thoughts. "Let go of past" means do not give much importance. The more importance you give it, the deeper impressions it makes and it will stay longer there to bother you. Just think of it as a dream. Dreams look so real while dreaming, that we don't realize it until we wake up. Similarly take past as a dream and wake up to this knowledge right this moment and get rid of those impressions.
Learn from the past and move forward. Don't hold on to it. Memory has a function to store impressions. so it is up to an individual what to store? You have got choice of storing what seems valuable to you. But, the wise man chooses to store life enhancing memories so that when he remembers them, his senses gets filled with joy and love and that exhilarating state where he feels lively... All non wanted impressions can easily be dropped out using OFS technique... Develop your intelligence to make it beneficial habit for the mind.