Tegami: Haikei Juugo no Kimi e

A music video from NHK's Minna no Uta featuring a song by Angela Aki and art by Fruits Basket's mangaka.

Based around 3 people as young children they sit down and write to their future self. They wish for the best and tell themselves to never give up faith or hope to always believe and stay strong even if everyone else puts them down. They just need to believe in thyself and they will be fine.

I won't give away much more then that, but at the end they thank their younger self's and how they gave the advice proudly accepting what happened and glad they wrote it. They just hope for their future self to be happy more than anything no matter what happens.