A/N: I chose not to directly state my take on what happened in the forest because it's something that I think Hermione would be doing her best to avoid articulating, even to herself, but, if you're curious, there are a number of articles about it online. The reason itself is well into M territory, though, so search it out at your own risk.
And thank you to my lovely brother, Kaayvan, for betaing this for me. If any of you are into Stargate: SG-1, you should totally go check out his writing.
The knowledge of what truly happened that night haunts her. It's like a shadow trailing behind her, persistently dogging her footsteps no matter how hard she tries to shake it off. When she least expects it, the memory will hit her afresh, bringing with it a fresh onslaught of guilt. Harry might have a naïve outlook about what happened in the forest, but Hermione knows better.
Even before Umbridge was dragged away by the centaurs, Hermione knew what they would do to the woman. But it was too late for her to intervene; there was no way for her to fight them all off. It would have been near impossible for two dozen armed witches and wizards to take them all on, let alone for Harry and Hermione to do it alone and wandless. Any such attempt would have been doomed to failure from the beginning. Faced with the choice of fruitlessly endangering more people or doing what she could to get out of there with Harry, she focused on trying to escape.
She recognises that there was nothing she could do about it; as soon as the centaurs caught sight of them, it was over. But absolution isn't as easy as knowing that it technically isn't her fault. Shame grips her whenever the memory of the retreating horde surfaces to torment her, and Ron's mocking hoof beat noises – clip-clop, clip-clop, clip-clop – echo in her brain long after she leaves his company. She found it funny when he first did it, but then the reality of the situation – of the reason for Umbridge's reaction – came crashing in and sobered her up.
Neither Ron nor Harry know. Ron has probably heard the lore before but forgotten it, while Harry – sweet, innocent, heroic Harry – has never been exposed to it. Her only wish now is that they never find out. The knowledge would torture Harry. In his mind, he's responsible for the safety of everyone around him, so he would feel that he had let her down if he knew. He would have risked his life to prevent it had he known, even knowing that there was no way to succeed. He is just that noble and self-sacrificing; the idea of prioritising self-preservation just doesn't occur to him when he's in the moment, and he sees things like jumping onto a troll's back or rushing down to fight a basilisk without any real backup as valid action plans. She can usually talk him into safer options if there are any, but that's as far as he will budge. If there isn't a safe option, or if he doesn't think it will work fast enough, he'll just throw caution to the wind and rush in headfirst like a bull in a china shop, not caring about what rules and safety measures he smashes through so long as he is doing his best to help. It's exasperating and confusing, but it's him.
And it's not her. She cares about people's wellbeing, but she has her limits. Aeroplane safety instructions always tell you to see to your own safety equipment before helping others; when the going gets tough, she lives by that as well. Although she's willing to accept danger when it's necessary if there's a chance of success, she will automatically prioritise her friends and herself if it reaches a critical point where all people can do is get out alive. There's only so long she can, ignoring the safety warnings, focus on saving other people before it reaches the breaking point and she really needs to see to her own safety.
After all, the Sorting Hat did mention that he would have considered putting her in Slytherin had she not been Muggle-born. She might be brave, but she still has a strong sense of self-preservation. She is logical enough to recognise when there's no chance of winning and to adjust accordingly. And, if faced with the thought experiment of choosing between saving a loved one or saving five strangers, she would pick her loved one every time.
So she knows – knows – that it wasn't her fault. Umbridge put her in an impossible position. As far as they knew, it was imperative to escape the castle, so Hermione jumped on the first even semi-plausible idea that came to mind. Grawp wouldn't have hurt Umbridge, not if Hermione told him not to. Had they gotten to him, nothing bad would have befallen the witch. It was the most practical, and only, choice at the time.
Still, although she knows there was nothing she could have done, an illogical part of her mind hates herself for not having tried harder. It picks up on the fact that part of her reticence to act was based on the fearful knowledge that she would, most likely, be taken as well, and twists and colours that until it comes out as cowardice. It takes the fact that she prioritised getting to Sirius over seeking help for Umbridge and spews that back out as being disinterested and uncaring. And it shines a torch on the fact that Harry would have run in, helpless or not, and distorts that until she feels immoral and weak in comparison.
She keeps reminding herself that none of that is the case at all, but the feelings are still there, and they're insidious enough to keep her awake at night weeks and months down the track. She tries to keep up a collected front, to react to the heavily annotated Potions textbook as she usually would, but she feels herself overcompensating. You took the self-interested way out when it came to Umbridge and to the Quidditch tryouts, her insecurities whisper at her, and so she does her best to find a moral standpoint and stick to it, however flimsy it might be.
Even as she blows up at Harry for following the book's instructions instead of Professor Slughorn's, for finally showing some sort of interest in school now that it's easy, she knows she's going about it the wrong way. A better tactic would be to use Harry's sudden Potions success to try to push him to focus more in other classes as well. But she took a stand, and she would feel horrible backing down from it now.
Fortunately, neither Harry nor Ron pick up on the reason behind her atypical behaviour; they're too preoccupied with Quidditch and girls and conspiracy theories to try to decode the ferocity of her reaction. To them, it's nothing but an unfair overreaction to something that she should embrace instead of condemn.
And she is glad for their ignorance. Over the course of their friendship, Harry has become her guidepost for what is right and good, the northernmost point on her moral compass. While she is ruthless and single-minded, and Ron is egocentric and insecure, Harry is innocently naïve and unwaveringly brave. Admitting to him the extent of the compromises she made last year would feel like letting him down, as if she were tarnishing herself in his eyes. After all they've been through together, that would send her reeling like nothing else could.
Sometimes, after all, it's harder to walk away from something that is unattainable than it is to fight for it and lose horribly. She had the foresight to see that it be pointless to try to overcome the centaurs, like a little kid trying to fell giants with weak little karate chops. But she still wishes there had been a way to do so without further exacerbating the situation, to make a run at them without then damning herself to the same fate, and Sirius and potentially Harry to death. She knows she was limited, but she desperately regrets not having had some way to act.
And that's what haunts her subconscious, twisting her dreams into a constant nightmare that plagues her even when she's awake.