They saw nothing of Marianne till dinner time, when she entered the room and took her place at the table without saying a word. Her eyes were red and swollen; and it seemed as if her tears were even then restrained with difficulty. She avoided the looks of them all, could neither eat nor speak, and after some time, on her mother's silently pressing her hand with tender compassion, her small degree of fortitude was quite overcome, she burst into tears and left the room.
"...I have never told you how this was brought on. We were within a few hours of eloping together for Scotland. The treachery, or the folly, of my cousin's maid betrayed us. I was banished to the house of a relation far distant, and she was allowed no liberty, no society, no amusement, till my father's point was gained. I had depended on her fortitude too far, and the blow was a severe one—but had her marriage been happy, so young as I then was, a few months must have reconciled me to it, or at least I should not have now to lament it. This however was not the case. My brother had no regard for her; his pleasures were not what they ought to have been, and from the first he treated her unkindly. The consequence of this, upon a mind so young, so lively, so inexperienced as Mrs. Brandon's, was but too natural. She resigned herself at first to all the misery of her situation; and happy had it been if she had not lived to overcome those regrets which the remembrance of me occasioned. But can we wonder that, with such a husband to provoke inconstancy, and without a friend to advise or restrain her (for my father lived only a few months after their marriage, and I was with my regiment in the East Indies) she should fall?..."
"... Surely this comparison must have its use with her. She will feel her own sufferings to be nothing. They proceed from no misconduct, and can bring no disgrace. On the contrary, every friend must be made still more her friend by them. Concern for her unhappiness, and respect for her fortitude under it, must strengthen every attachment. Use your own discretion, however, in communicating to her what I have told you...."
To give the feelings or the language of Mrs. Dashwood on receiving and answering Elinor's letter would be only to give a repetition of what her daughters had already felt and said; of a disappointment hardly less painful than Marianne's, and an indignation even greater than Elinor's. Long letters from her, quickly succeeding each other, arrived to tell all that she suffered and thought; to express her anxious solicitude for Marianne, and entreat she would bear up with fortitude under this misfortune. Bad indeed must the nature of Marianne's affliction be, when her mother could talk of fortitude! mortifying and humiliating must be the origin of those regrets, which SHE could wish her not to indulge!
Her exertions did not stop here; for she soon afterwards felt herself so heroically disposed as to determine, under pretence of fetching Marianne, to leave the others by themselves; and she really did it, and THAT in the handsomest manner, for she loitered away several minutes on the landing-place, with the most high-minded fortitude, before she went to her sister. When that was once done, however, it was time for the raptures of Edward to cease; for Marianne's joy hurried her into the drawing-room immediately. Her pleasure in seeing him was like every other of her feelings, strong in itself, and strongly spoken. She met him with a hand that would be taken, and a voice that expressed the affection of a sister.
"Your sister," he continued, "has suffered dreadfully. Mrs. Ferrars too—in short it has been a scene of such complicated distress—but I will hope that the storm may be weathered without our being any of us quite overcome. Poor Fanny! she was in hysterics all yesterday. But I would not alarm you too much. Donavan says there is nothing materially to be apprehended; her constitution is a good one, and her resolution equal to any thing. She has borne it all, with the fortitude of an angel! She says she never shall think well of anybody again; and one cannot wonder at it, after being so deceived!—meeting with such ingratitude, where so much kindness had been shewn, so much confidence had been placed! It was quite out of the benevolence of her heart, that she had asked these young women to her house; merely because she thought they deserved some attention, were harmless, well-behaved girls, and would be pleasant companions; for otherwise we both wished very much to have invited you and Marianne to be with us, while your kind friend there, was attending her daughter. And now to be so rewarded! 'I wish, with all my heart,' says poor Fanny in her affectionate way, 'that we had asked your sisters instead of them.'"
The Dashwoods were two days on the road, and Marianne bore her journey on both, without essential fatigue. Every thing that the most zealous affection, the most solicitous care could do to render her comfortable, was the office of each watchful companion, and each found their reward in her bodily ease, and her calmness of spirits. To Elinor, the observation of the latter was particularly grateful. She, who had seen her week after week so constantly suffering, oppressed by anguish of heart which she had neither courage to speak of, nor fortitude to conceal, now saw with a joy, which no other could equally share, an apparent composure of mind, which, in being the result as she trusted of serious reflection, must eventually lead her to contentment and cheerfulness.
[...]"They have borne more than our conduct.—Do not, my dearest Elinor, let your kindness defend what I know your judgment must censure. My illness has made me think— It has given me leisure and calmness for serious recollection. Long before I was enough recovered to talk, I was perfectly able to reflect. I considered the past: I saw in my own behaviour, since the beginning of our acquaintance with him last autumn, nothing but a series of imprudence towards myself, and want of kindness to others. I saw that my own feelings had prepared my sufferings, and that my want of fortitude under them had almost led me to the grave. My illness, I well knew, had been entirely brought on by myself by such negligence of my own health, as I had felt even at the time to be wrong. Had I died,—it would have been self-destruction. I did not know my danger till the danger was removed; but with such feelings as these reflections gave me, I wonder at my recovery,—wonder that the very eagerness of my desire to live, to have time for atonement to my God, and to you all, did not kill me at once. Had I died,—in what peculiar misery should I have left you, my nurse, my friend, my sister!—You, who had seen all the fretful selfishness of my latter days; who had known all the murmurings of my heart!—How should I have lived in YOUR remembrance!—My mother too! How could you have consoled her!—I cannot express my own abhorrence of myself. Whenever I looked towards the past, I saw some duty neglected, or some failing indulged...."
"When the dessert and the wine were arranged, and Mrs. Dashwood and Elinor were left by themselves, they remained long together in a similarity of thoughtfulness and silence. Mrs. Dashwood feared to hazard any remark, and ventured not to offer consolation. She now found that she had erred in relying on Elinor's representation of herself; and justly concluded that every thing had been expressly softened at the time, to spare her from an increase of unhappiness, suffering as she then had suffered for Marianne. She found that she had been misled by the careful, the considerate attention of her daughter, to think the attachment, which once she had so well understood, much slighter in reality, than she had been wont to believe, or than it was now proved to be. She feared that under this persuasion she had been unjust, inattentive, nay, almost unkind, to her Elinor;—that Marianne's affliction, because more acknowledged, more immediately before her, had too much engrossed her tenderness, and led her away to forget that in Elinor she might have a daughter suffering almost as much, certainly with less self-provocation, and greater fortitude. "
There is a certain amount of ambiguity in these mentions of fortitude: Colonel Brandon's use of the term (both Chapter 31 passages) is so highly colored by his peculiar experience that it opens the question of how well grounded his sense of what fortitude requires might be; John Dashwood's reference to fortitude (Chapter 37) is not really worth anything; and the narrator pokes fun at Elinor's fortitude at least once (Chapter 35). It is almost certainly also significant that every mention by male characters of fortitude in a woman is about her relations to men, whereas every use by the narrator is about her relations to herself (in particular, keeping her feelings in check).
Comparing to other novels, 'fortitude' shows up once (in a clearly ironic usage) in Lady Susan, once in Pride and Prejudice, three times in Northanger Abbey, four times in Emma. All of these usages seem quite minor; there isn't a great deal of moral weight attached to them. The interesting comparison is with the two novels that can be seen as addressing fortitude most directly: 'fortitude' appears six times in Persuasion, at least most of which do put considerable moral weight on the term, and not at all in Mansfield Park. Mansfield Park uses the term 'constancy' and related terms instead; in all the other novels where the term carries moral import (it usually doesn't), 'constancy' just means faithfulness in a relationship, but Fanny Price's constancy, which, while not referred to extensively rises to the front at several key points in the plot, is not constancy in this sense at all, but is at least much more like what is called fortitude elsewhere.
So is this just a peculiar accident ,or is there, perhaps, some difference of association? Perhaps 'fortitude' suggests to Austen a power of self-mastery with regard to the behavior by which one expresses one's feelings -- this arguably would differentiate Elinor and Anne on one side from Fanny on the other, since all three exhibit constancy of a sort, but Fanny doesn't have the relatively iron control of Elinor or Anne, as shown by the famous fact that we find her crying fourteen times in the novel.