Basically it‘s simple:
Himeno and him were lost causes already at the age of 10 but they were bound to be together, to at least ease the pain a little bit.
She had to move which led to losing this connection. She talked about being a smartass at the age of 10 only to emphasize that this later on probably led to some foolish (naive) behavior, resulting in her pregnancy. I don‘t assume the husband was the father of the child as they only married a year after the child was born. Maybe she was raped. And in her misery she sent a letter to her only real friend, that was always as miserable as her and understood her. And she told him in a way, only someone that knows her would understand to then get a reply which causes her to lose all hope. Cuz she couldn‘t speak openly about the pregnancy/ being raped as she did not have the courage. With her whole misery being somewhat bearable knowing she‘s not alone, that time has passed now so she focused all her resentment to him instead of the people that actually deserve it.
it‘s good that at least the point is taken that she is actually insane in her behavior and this isn‘t „deserved“ in any way. It should never be. I mean give him a call or something and don‘t end your life on ONE letter. I don‘t know if it‘s a problem with the translation but as it‘s saying „calls“ as if she tried to reach out to him several times, that is really a big exaggeration.
But man this hits hard. Also getting older and realizing how little friends remain over time just makes this manga worse, hitting dem feels