One might wonder why a pragmatist must step in to be the heroine, because this is a fairy tale and one would think the hero would be bright enough to pick someone who actually believes in true love. But Hermione makes do, caught up in the euphoria, and she doesn't know when the story became a cage.
She knows that she is not a heroine. Heroines are pretty things to be rescued. Only the girl Harry rescued properly got married in trainers and has an in-law with a vulture hat, and this isn't how things should be. And she's a better choice anyway, if one makes little lists and tallies up pros and cons. She had her perfect wedding to her perfect husband, so things are okay. Or at least she tells herself that things are okay, because she is tethered now and can't hope to escape. The weight of white dresses and dreams presses down on her, so that now, torn from her proper place, she can't even scream.
Now she sits in the house little girls dream of when they play with their dolls, with the handsome prince who's just as miserable as she is, and she wishes for a thousand different might-have-beens. She wishes the air wasn't stale. She wishes she could pull Harry to her level, because she can't be the good woman behind the good man. She will be his friend, and love him and cherish him and face down all dangers for him, but this is not her place and her power has slipped away.
There is a difference between heroine and sidekick. Sometimes she hates Ginny for shirking her duty, and sometimes she hates Ron for joining the Cannons and slipping away from the trap. Once she writes to him and tells him that she and Harry never fight, that they never speak because whoever heard of the hero and heroine arguing about the dishes, and he asks her what's the fun in that.
She has no answer. Love is mundane and evolving, and the rarified form that chains her and Harry can't exist in the real world. The worst part is that now, when she truly needs help, no one will ever come to her aid. Whoever heard of someone being rescued from happily ever after?