In Ch. 46 Marianne says:

"..if I could be allowed to think that he was not always acting a part, not always deceiving me; -- but above all, if I could be assured that he never was so very wicked as my fears have sometimes fancied him, since the story of that unfortunate girl" --
My peace of mind is doubly involved in it; -- for not only is it horrible to suspect a person, who has been what he has been to me , of such designs, -- but what must it make me appear to myself? What in a situation like mine, but a most shamefully unguarded affection could expose me to -- "

In response, Elinor "related simply and honestly the chief points on which Willoughby grounded his apology; did justice to his repentance, and softened only his protestations of present regard."

In Ch.47, Elinor tells her Mother:

Reflection had given calmness to [Elinor's], and sobered her own opinion of Willoughby's deserts; she wished, therefore, to declare only the simple truth, and lay open such facts as were really due to his character,

"Mrs. Dashwood did not hear unmoved the vindication of her former favourite. She rejoiced in his being cleared from some part of his imputed guilt; she was sorry for him; she wished him happy. .....
Nothing could do away the knowledge of what [Marianne] had suffered through his means, nor remove the guilt of his conduct towards Eliza."

Later in the same chapter Elinor speaks of Willoughby's selfishness, and says:

"At present, ...he regrets what he has done. And why does he regret it? Because he finds it has not answered towards himself. It has not made him happy.

"One observation may, I think, be fairly drawn from the whole of the story -- that all Willoughby's difficulties have arisen from the first offence against virtue, in his behaviour to Eliza Williams. That crime has been the origin of every lesser one, and of all his present discontents."