She can see now that she wasn't right for him, or him for her. But they tried, and they loved, and she couldn't take it. She is still splintered by the war, by losing her parents. Everyone deals with things in different ways- she knows this- but it took trying to play the part of someone she is not to realise that. It took breaking her best friend's heart.
Hermione Granger has always been the girl behind a book, more at home in a library or in a fictional world, than among conversations of quidditch or boys or hobbies. That didn't change when she went to Hogwarts and made friends that didn't whisper behind her back, she just became more.