Writing a letter to your past self helps you to understand and reconcile with your past self better. It also reminisces on prior accomplishments and memories. Here is more information on why you should write a letter to your past self: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10927754/#:~:text=Writing%20to%20past%2Dself%20decreased,letter%20to%20their%20past%2Dself.