From what I've observed...people who are painfully shy often don't outgrow it. It's one thing to ride a horse, it's another to be the complete and absolute center of attention in a negative fashion...and after your aunt has just abused you (again, loudly, in front of non-family members).

In standing up to dance, she isn't the center of attention.

The person who angers me here is Edmund, who, though this entire chapter, comes off as less than the hero Fanny sees him as. (Discussed elsewhere.) Mary, however, comes to her defense whether out of sincere sympathy or for other reasons, she's still the "hero" of the moment for Fanny.