Recall that in the early chapters, Emma herself has a guilty conscience concerning her treatment of Jane from childhood on, and still refuses to mend her attitude.

If Emma had had any kind of proper feeling toward Jane, even of disinterest, she would have noticed Jane's increased reserve and wondered about it -- just as Mr. Knightley did. But she let her lifelong jealousy of Jane's superior accomplishment compared to her own draw her into a disgusting misinterpretation (worse, revealing it!) of the increased reserve.

The fact that Jane made a possibly foolish mistake does not in the least absolve Emma of her guilt.

...cringing even lower behind my wall...