As people grow and change, sometimes they wish they could be more like their younger self–be that because they miss the wonder they had in the world, or because they’ve lost their faith in people. Other times we wish we could get word from the future that things turn out all right. A popular way of expressing those wishes is in writing a letter to yourself–either past or future.
As an exercise: do just that. Write a letter to your character. Either to tell them about the things coming up in the future, or to remind them of the good stuff they came from. Perhaps you want to apologize to them for all the things they go through in the course of the story, or explain how you really feel about them.
Alternately, you can have your character complete the exercise. Think about things like if they’d want to write to their younger self, or to their future one. When writing to their younger self, consider the things they struggled to get through. When writing to the future version, what qualities do they admire in themselves currently?