"....with common sense... I am afraid I have had little to do." is very promising. And that is before she finds out she was wrong again about the object of Harriet's feelings. After all the revelations that the truth on that matter brings, she beats herself up very thoroughly, though quite deservedly, for her past errors and sees how wrong she was. Funny, isn't it, how her former certainty that Harriet is a gentleman's daughter, which would make her somewhat less unacceptable as a match for Mr. Knightley, has evaporated, without the arrival of any new evidence on the matter. And how Robert Martin has suddenly become "the unexceptionable young man who would have made [Harriet] happy and respectable in the line of life to which she ought to belong", when he used to be a degradation.