I think of part of Marianne's reaction right after Willoughby leaves town.
Marianne would have thought herself very inexcusable had she been able to sleep at all the first night after parting from Willoughby. She would have been ashamed to look her family in the face the next morning, had she not risen from her bed in more need of repose than when she lay down in it.
I should clarify that in case my posts make it comes across like I'm professing truths that engraved in stone or something, I am just expressing my opinion. :)
And I agree with you on your last statement, Anselm, that the passage above is related to the romantic sensibility that she has adopted. But she does seem to be conscious of how she appears to her family. Can go either way, whether she's trying to appear to family as stricken (which would be a bit of being a drama queen, IMO) or merely wanting to play out her sensibility and she would be disappointed if the sensibility she has adopted doesn't actually play out so that she is ill. Not sure it makes a difference, IMO.