Living in the past harms the present and consequently the future. Effects might be different but the underlying cause is not making full use of the present.
Someone might have had a memorable past but letting life flow in that acute sense of nostalgia is not giving justice to present life.
Life poses duality here. Though we are supposed to look at life in a holistic perspective, it’s also said we have to be totally present in the present. Its also said that we should not thrive on our past but past is the place that all your upbringing, your cultural understanding comes from. At this point, it becomes necessary to dissociate these concepts from each other.
So, the lesson is probably to forget the achievements and remember the lessons; forget the unnecessary burden from the past and remember the inner spirit, use the past, don’t ride on it.