Lady Elliot had been an excellent woman, sensible and amiable; whose judgement and conduct, if they might be pardoned the youthful infatuation which made her Lady Elliot, had never required indulgence afterwards.--She had humoured, or softened, or concealed his failings, and promoted his real respectability for seventeen years; and though not the very happiest being in the world herself, had found enough in her duties, her friends, and her children, to attach her to life, and make it no matter of indifference to her when she was called on to quit them. (1.7)
Some mistakes can be fixed, but not a foolish choice of marriage partner – unless one is lucky, like Mr. Elliot, to be conveniently widowed when one's spouse becomes a burden. Lady Elliot seems a bit like Mrs. Smith, in that she tries to make the best of a bad situation.
She often told herself it was folly, before she could harden her nerves sufficiently to feel the continual discussion of the Crofts and their business no evil. (4.10)
Here Anne uses accusations of folly as a buffer between herself and her emotions – by telling herself that what's she's feeling is stupid, she tries to convince herself not to feel it at all.
Now, how were his sentiments to be read? Was this like wishing to avoid her? And the next moment she was hating herself for the folly which asked the question. (7.30)
Folly here seems to mean pointlessness – since she doesn't think there's any way they'll get back together, there's no reason for her to think so much about what he's thinking. Self-hatred seems a rather strong reaction, though – why is she so mad at herself for thinking this?
"That is the woman I want," said he [Captain Wentworth]. "Something a little inferior I shall of course put up with, but it must not be much. If I am a fool, I shall be a fool indeed, for I have thought on the subject more than most men." (7.41)
Here's yet another version of folly – acting without thinking. Some characters, however, seem to be foolish even when they do think (Sir Walter comes to mind), because what they think is so ridiculous.
As to the sad catastrophe itself, it could be canvassed only in one style by a couple of steady, sensible women, whose judgements had to work on ascertained events; and it was perfectly decided that it had been the consequence of much thoughtlessness and much imprudence; that its effects were most alarming, and that it was frightful to think, how long Miss Musgrove's recovery might yet be doubtful, and how liable she would still remain to suffer from the concussion hereafter! (13.20)
"Thoughtlessness" as folly again – but is the accident entirely Louisa's fault? (If so, could it still be called an accident?)
"The notions of a young man of one or two and twenty," said he [Mr. Elliot], "as to what is necessary in manners to make him quite the thing, are more absurd, I believe, than those of any other set of beings in the world. The folly of the means they often employ is only to be equalled by the folly of what they have in view." (15.20)
The folly of what young men have in view, what they want, is success in society – which means that Mr. Elliot is implying that seeking society's approval is foolish. Or perhaps he's only saying that a young man's idea of what society wants is incorrect, and therefore foolish.
She hoped to be wise and reasonable in time; but alas! alas! she must confess to herself that she was not wise yet. (19.25)
Apparently being wise is like being pregnant – either you are or you aren't.
[Mrs. Smith speaks] "I was very young, and associated only with the young, and we were a thoughtless, gay set, without any strict rules of conduct. We lived for enjoyment. I think differently now; time and sickness and sorrow have given me other notions." (21.63)
As Mr. Elliot did when he was speaking to Anne of young men's absurdity, Mrs. Smith associates foolishness with being young. But she suggests that growing up isn't enough to make someone stop being foolish – it takes a dose of suffering to wise up.
Captain Wentworth, with five-and-twenty thousand pounds, and as high in his profession as merit and activity could place him, was no longer nobody. He was now esteemed quite worthy to address the daughter of a foolish, spendthrift baronet, who had not had principle or sense enough to maintain himself in the situation in which Providence had placed him. (24.2)
While it seems to be the right of the upper classes to be foolish with few social consequences (look at the popularity of Lady Dalrymple), money is one area where foolishness is truly dangerous, and can even damage one's class standing.
There was nothing less for Lady Russell to do, than to admit that she had been pretty completely wrong, and to take up a new set of opinions and of hopes. (24.3)
Lady Russell was wrong, but was she foolish? (What's the difference?)