I agree that Elinor was swayed by his charm and also by the stress and fatigue she felt at the time Willoughby talks to her. Once she gains a bit of perspective, though, and is away from his influence, I think she is pretty clear about how she feels about his 'confession' when she speaks to Marianne in Ch. 47:

His expensiveness is acknowledged even by himself, and his whole conduct declares that self-denial is a word hardly understood by him. His demands and your inexperience together on a small, very small income, must have brought on distresses which would not be the less grievous to you from having been entirely unknown and unthought of before. Your sense of honour and honesty would have led you, I know, when aware of your situation, to attempt all the economy that would appear to you possible; and perhaps, as long as your frugality retrenched only on your own comfort, you might have been suffered to practise it, but beyond that -- and how little could the utmost of your single management do to stop the ruin which had begun before your marriage? -- beyond that , had you endeavoured, however reasonably, to abridge his enjoyments, is it not to be feared, that instead of prevailing on feelings so selfish to consent to it, you would have lessened your own influence on his heart, and made him regret the connection which had involved him in such difficulties?"

Marianne's lips quivered, and she repeated the word "Selfish?" in a tone that implied "Do you really think him selfish?"

"The whole of his behaviour," replied Elinor, "from the beginning to the end of the affair, has been grounded on selfishness. It was selfishness which first made him sport with your affections -- which afterwards, when his own were engaged, made him delay the confession of it, and which finally carried him from Barton. His own enjoyment, or his own ease, was, in every particular, his ruling principle.

"It is very true. My happiness never was his object."

"At present," continued Elinor, "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."

I think that last line sums it up perfectly. Elinor does not want to hold a grudge against Willoughby forever, and she feels sorry that he behaved the way that he did more than she feels sorry for him, but the bottom line is that Willoughby only regrets what has made him unhappy, and what has lessened his own enjoyment. His regret does not really extend to anyone or anything else, and it may not even be permanent (At presesent...) He doesn't care about anyone other than himself and he never will.