Why do some people struggle to live without clinging to their past? (People who think everything was better in the past, and there's no harmony between what was lived and the here and now.)

Living through past memories and nostalgia is a characteristic of many people. They might be individuals who focus on valuing what they've lost: youth, loved ones, the dynamics of a particular time, etc. The perception of loss induces sadness, and nostalgia is a form of sadness. It is indeed good to relive experiences of love with loved ones, but it is not beneficial to dwell on the past and recreate the sadness. It is more important to project oneself into the future with hope, enthusiasm, and energy.