The fact that Haru feels like she's some type of replacement for Tsugami is very conflicting to me. I mean yes, Midori did say that Haru is different than her mom and she appreciates her for her, but at the same time the author draws them looking exactly the same and this doesn't help. I was sad reading the scene with the letter, but then Haru pops up and it's like "Tsugami died but Haru is exactly like her, so let's pretend everything's ok". So it feels like a metaphor for trying to deny reality and repress the feelings.
I can see where that reading comes from, but it’s pretty much the opposite of what the text says. (You may not find what the text says convincing, of course.)
That scene with the letter is not a preliminary to Haru and Midori being together, but to their parting—Midori isn’t denying reality, she’s accepting the loss of Tsugumi and moving on, only without the immediate presence of Haru but with the bond that the two of them have created together still intact.
The series begins with (and repeats several times) Midori thinking about how she hates spring, which is supposed to be about fresh starts but only reminds her of the loss of Tsugumi. By the end she welcomes spring, and the Haru that returns to her is completely different in style and dress (and height) than the Tsugumi she remembers.
And I think the whole point is that Haru isn’t “exactly like” Tsugumi. In fact, she’s kind of the opposite—Tsugumi was warm and affectionate on the outside but hid her true self (the one who knew she would leave) from Midori, while Haru is Miss Tsundere—mocking Midori to her face (“You’re such a weirdo”) but deeply bonded to her.
So in the sense that Midori had previously only cared for Tsugumi, sure, Haru is technically a “replacement,” but I don’t see any repression or denial in their relationship, just acceptance.
