Peg Kerr's Journal

See my answer to cakmpls here;
also, I would correct myself on assessing Marianne at the beginning of the book, actually. Marianne begins the book being callous of the feelings of others. After her illness, she reflects upon what she has learned:

"I am not wishing him [Willoughby] too much good," said Marianne at last with a sigh, "when I wish his secret reflections may be no more unpleasant than my own. He will suffer enough in them."

"Do you compare your conduct with his?"

"No. I compare it with what it ought to have been; I compare it with yours."

"Our situations have borne little resemblance."

"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. Everybody seemed injured by me. The kindness, the unceasing kindness of Mrs. Jennings, I had repaid with ungrateful contempt. To the Middletons, the Palmers, the Steeles, to every common acquaintance even, I had been insolent and unjust; with an heart hardened against their merits, [note the heart of stone metaphor used here!] and a temper irritated by their very attention. -- To John, to Fanny, -- yes, even to them, little as they deserve, I had given less than their due. But you, -- you above all, above my mother, had been wronged by me. I, and only I, knew your heart and its sorrows; yet, to what did it influence me? -- not to any compassion that could benefit you or myself. -- Your example was before me: but to what avail? -- Was I more considerate of you and your comfort? Did I imitate your forbearance, or lessen your restraints, by taking any part in those offices of general complaisance or particular gratitude which you had hitherto been left to discharge alone? No; -- not less when I knew you to be unhappy, than when I had believed you at ease, did I turn away from every exertion of duty or friendship; scarcely allowing sorrow to exist but with me, regretting only that heart which had deserted and wronged me, and leaving you, for whom I professed an unbounded affection, to be miserable for my sake."

Marianne in this scene clearly sees the difference between the heart of flesh and the heart of stone. The pain of Willoughby's betrayal has tutored her in the difference, and she clearly ends the book making a deliberate choice to turn from one path to the other. Again, clearly, the key is recognizing the selfishness underneath of the heart of stone.