She was at the wedding, that was the worst part.
He could have dealt with it if she wasn't; he could have forgotten about how it all went wrong, and just lost himself in the moment, maybe even enjoyed it. But of course she was at the wedding, and it killed him.
His wife didn't know that she wasn't his original choice. How could she know? No- one knew, even Hermione, who was supposedly a relationship genius.
As if.
But as he stood there at the reception, watching her dance (and that dance, the way she just span around, eyes closed, arms out, same as she did when she was sixteen, carefree and innocent) he couldn't help but re- live every memory he had of her... and every one just made that hole in his heart a little bit bigger.
She didn't know, even. Only he knew how he befriended his now- wife to get to her in the first place; how it all went horribly wrong and he ended up with the wrong girl, but there was too much happiness at stake to risk (plus the fact that she was the daughter of the people who were practically his parents, and the sister of his best friend, and he couldn't lose them).
But he could never forget it, and he knew that it was too late but he could still wish.
Harry could still wish that it wasn't Ginny he was married to; that it was Luna, it always had been, and nothing could ever change that.
But she danced there, painfully (painlessly? For her, at least,) unaware, and Harry knew that it was just too late to go back now.