Despite how terribly she misses Ron, Harry really is one of her absolute best friends, and he's always been that way. If she had to be stuck in this situation with anyone, Hermione's glad that it's Harry, someone she could honestly trust with her life. She knows she can trust him with it, because she has before and she's always come out alive.
Harry undoubtedly knows how upset she is by Ron's leaving, and when they move for the first time without Ron, that's when she honest to God thinks it's final. If Ron wanted to come back (however slim a chance of that there was), he was going to try that spot where they had been when he left. They wouldn't be there, and he would leave. He would leave because they'd forgotten about him, and it would be all her fault since she went along with Harry's nudging hints about changing campsites. Their moving campsites signifies leaving Ron behind, and Hermione just can't deal with that on top of the deaths, her parents' Obliviate-ing, the War, not going back to Hogwarts, and not knowing how everyone is doing. She doesn't know if half of the people she knows are alive or dead.
It gets hard, and Harry & Ron were her glue; they kept her together. Harry is a brilliant guy, he is. The Chosen One and all that. Harry's fantastic, but it doesn't make him Ron.
Curling up on the steps is such a pitiful thing to do, but she does it because maybe she wants Harry to realize that he can't just let her implode on all of this. She's going to go insane if they keep ignoring it much longer. Ron is gone, and they need to stop ignoring it and start talking about it. Her eyes are closed tightly when she hears Harry come in. It would only be a bother to look up at him, because as of late they've been barely speaking to each other. It could just be the work of the Horcrux that infers with their moods more than they'd like to admit, but she suspects it's also because Harry simply doesn't know what to say to her that could make this better. She feels worse when she's wearing the Horcrux, as though nothing is ever going to make Ron's leaving better ever again. Feeling slightly at the heavy pendent hanging around her neck, she barely acknowledges that she's wearing the Horcrux. Everything just hurts nowadays, and it's gotten to the point where nothing is alright anymore, because Ron left them and Ron isn't ever coming back.
She keeps her eyes closed, wishing that one day she's going to be strong enough to tell Harry that they can't keep pretending that Ron never left. She wants to tell him already, and now is the perfect time, but Hermione really doesn't know how. She's not brave enough to just bring it up, but they can't go on like this for much longer without it ending in a catastrophic result. As she thinks this, she hears Harry cross the room. Unbidden hopes that he wants to fix this rift jump to her mind. She barely opens her eyes and peers through her lashes to see that he's holding out a hand to help her up from the stairs.
She wants to open her mouth and say something about how maybe they should try to talk about Ron to make this aching gap go away, but the atmosphere doesn't call for talking.
They start to dance slowly around the room, and it doesn't make everything better by any means, but Hermione knows that it's a start to closing the gap she knows they both feel.