It's clear that the only reason he came was because he thought Marianne was on her death bed.

"Had I known as much half an hour ago -- But since I am here" -- speaking with a forced vivacity as he returned to his seat -- "what does it signify?

What would he have done if he had known as much half an hour ago, - not come? He wasn't prepared at all for her recovery;

I thought of her to-day as really dying, it was a kind of comfort to me to imagine that I knew exactly how she would appear to those who saw her last in this world. She was before me, constantly before me, as I travelled, in the same look and hue."

He has rehearsed for a deathbed scene and is now totally out of his element.

When Elinor cuts him short by saying that he shouldn't worry because Marianne has already forgiven him, he's frustrated.
This is not what he wants.

"Then she has forgiven me before she ought to have done it. But she shall forgive me again, and on more reasonable grounds."

Hmmm. What are these reasonable grounds?
-That Marianne accepts his behaviour (which was such a torture to him ;-) as something unavoidable.
-That Marianne sees why Willoughby had to do what he did - and on those reasonable grounds, forgive him.

I noticed that Elinor's comments in between are very leading which directs Willoughby in what to say. Otherwise he's actually very unprepared.
He has only described how it all tortured him. How he suffered!!
And there is this laughable comment (well actually all are).
It is about the visit to Barton cottage before he left for London, and the struggle he went through whether to go or not;

To see Marianne, I felt would be dreadful, and I even doubted whether I could see her again, and keep to my resolution. In that point, however, I undervalued my own magnanimity, as the event declared; for I went, I saw her, and saw her miserable, and left her miserable -- and left her, hoping never to see her again."
His magnanimity. LOL!!