Our future signed in letter

What if everyone around you was able to come to terms with the past, but not yourself? This is the situation in which Mikasa finds herself after she hastily fled with Eren's head. Now, she has to come to terms with her life on Paradis. In the thick of her sadness and depression, she finds a stop in a friendship kept in pen that may determine her future.
Events after Chapter 139.
Mikasa is 22 / 25 years old, Levi 36 / 39 years old , and Falco and Gabi are 15 / 18 years old.
Setting is in the beginning of 1917 and middle of 1920.
The prolog happens three years after Mikasas escape with Erens head. And the beginning of the first chapter happens three years after the prolog or six years after the rumbeling.