Robbin said:
IMO the thought of Marianne marrying Col Brandon is for Willoughby like loosing a prize trophy to his most hated rival.
Willoughby regrets throwing Marianne away but he particularly hates the thought of loosing her to Col Brandon because he once had him beat. Perhaps it is jealousy and resentment, I do not know for sure, but people do sometimes really hate those who make them look bad and compared to Col Brandon, Willoughby really looks bad.
I totally agree with this analysis.
Something that strikes me in CH.44 is that W. doesn't mention the duel at all - not a word. Given how voluble his 'confession' has been, including many statements deeply unflattering to himself, I think this omission is significant. It suggests to me that Willoughby felt humiliated in some way by the encounter. If the man who humiliated him were later to 'get the girl' that would really be rubbing salt in the wound - so much so that W. is torturing himself just imagining it.