I agree that Jane Fairfax is not so sublime, JA did not write of sublime, flawless characters, the best of her heroes and heroines are human and had faults, and learned to improve themselves.

But it would be very unfair to compare her with Lucy Steele. Lucy never loved Edward Ferrars, was only interested in his mother's money, knew that Edward regretted their secret engagement but did not release him from it.

Jane was "excessively attached" to Frank, she found his liveliness "bewitching" and allowed her affection to overpower her judgment when she consented to the secret engagement. Another serious character who was "bewitched" by the lively manners of a member of the opposite sex was Darcy feeling bewitched by Elizabeth Bennet.

It would also be quite unfair to compare Jane with Marianne. Marianne made herself ill after her disappointment over Willoughby, in spite of everything that Elinor did and said to comfort and counsel her. Jane had nobody to comfort or counsel her. In fact, her ill-health had begun when she consented to the secret engagement (re chap 20), because she had been suffering in her conscience and feeling guilty about it. She visted Highbury partly to be there when Frank came to visit, but her wish to be with her aunt and grandmother was also genuine -- re chap 20 -- "With regard to her not accompanying them to Ireland, her account to her aunt contained nothing but truth, though there might be some truths not told."