Maybe that's the core of the difference between the two, and the reason why Marianne can improve while Willoughby can't - at least by this stage. Her repentance is completely real. She is thoroughly ashamed of herself, and - more to the point - thoroughly sorry for the hurt she has caused others, particularly Elinor. In Ch.37 she is geniunely interested in someone else, as is clearly demonstrated by her reaction to Elinor regarding the revelation of Edward's longstanding engagement to Lucy:

"How long has this been known to you, Elinor? has he written to you?"

***

...Marianne's eyes expressed the astonishment which her lips could not utter. After a pause of wonder, she exclaimed --
"Four months! -- Have you known of this four months?" --

***

"What! -- while attending me in all my misery, has this been on your heart? -- and I have reproached you for being happy!" --

***

"Four months!" -- cried Marianne again. -- "So calm! -- so cheerful! -- how have you been supported?"

***

"Four months! -- and yet you loved him!"

***

"Oh! Elinor," she cried, "you have made me hate myself for ever. How barbarous have I been to you! -- you, who have been my only comfort, who have borne with me in all my misery, who have seemed to be only suffering for me! -- Is this my gratitude! Is this the only return I can make you? Because your merit cries out upon myself, I have been trying to do it away."

Willoughby demonstrates no equivalent to this. His concern is all with himself. So far, and to the extent that he consciously recognises this, he is "genuine" - but what an inadequate genuineness! I believe that his utterances show him to be truly incapable of any meaningful empathy with anyone else, much less any capacity for self-improvement. All of his confession in Ch.44 concerns himself. I find it quite striking that he never actually expresses unmitigated concern for Marianne. He does say "God be praised!" in response to the servant's declaration that she is out of danger. But when he presses Elinor for confirmation, she replies "We hope she is". Only "we hope"? If he'd been truly in love with Marianne, as opposed to with his idea of being in love with his idea of her, surely he would have replied along the lines of "What do you mean, "We hope"? Are you not certain? Let me see her this instant!"

Instead, what does he say? That enigmatic "Had I known as much half an hour ago --". And what eloquent dashes! What would he have gone on to say? Whatever that might have been, he does go on to indulge in is some relatively light-hearted (although anything but trivial) banter - a tad inappropriate, no? Not once in the ensuing conversation does he express any kind of regret, or even reference to the possibility, of having been, even partly, the cause of her being at death's door. I know, I know... putrid fever... nothing to do with him... But this ought to have been grist to the mill of his finely honed and innate sense of melodrama, if not to any real sense of responsibility to someone else. His report of his reaction to the news of Marianne's grave illness is:

What I felt on hearing that your sister was dying -- and dying, too, believing me the greatest villain upon earth, scorning, hating me in her latest moments -- for how could I tell what horrid projects might not have been imputed? -- One person I was sure would represent me as capable of anything. What I felt was dreadful!

I, I, I - hardly anything of actual concern for Marianne's state! What a contrast to Marianne's real, heartfelt concern for Elinor in the quotes above!

I think the essence of his response is a heightened self-awareness, but little or no inclination to change himself. This is shown by two of his statements near the end of his "confession". One is:

"As to that," said he, "I must rub through the world as well as I can. Domestic happiness is out of the question.

This attitude must throw the possibility of any moral self-improvement on the part of Willoughby into serious doubt. The other statement is his parting shot: "...I have most injured I can least forgive". No hint of contrition for this perverted attitude - in fact, almost a defiant pride in it. What a miserable creature! If he can't even pretend to be sorry for such an extreme and vile outlook, what hope has he of even truly recognising, let alone bettering, his fundamental attitude towards other people?

I must still respectfully disagree with the term "drama queen" as applied to Elinor. Fanny's reaction to Nancy's revelation of Lucy's and Edward's engagement - now there's the reaction of a true drama queen. She wants the world to know what she's going through. But Marianne, frankly, doesn't give a damn, my dear. Her sole concern is to behave in a way that is as consistent as possible with her beliefs - entirely laudible, I'd say.