Col. Brandon's confession in ch. 31 is one of my favourites in the novel. Although Elinor--and therefore we--suspected before now that he had a disastrous love affair in his past (particularly from his speech at the end of Ch. 31) but the full extent of his suffering comes as a surprise nevertheless.
The Broadview edition editor, Kathleen James-Cavan, points out how Brandon refers to himself as an 'awkward' narrator, but that he is, in fact, very effective. From a man who has hitherto been 'silent and grave', this is very emotionally charged, and the dashes, circling back, and breaking off with "How blindly I relate!" or even breaking off and being unable to keep speaking, keep the emotion running high.
I often think that I would have liked to have Marianne hear this from him firsthand, and watch her react to the news that he tried to elope with his first love, that he signed himself off into foreign service so that the woman he loved might find a chance at happiness in her marriage, and then that he fought a duel with Willoughby!
I also really find my heart aches for him in what he has had to go through in making the decision to come forward with this information. Once this is all revealed to Elinor, it is clear he has been debating whether he ought to tell her for some time, and even almost started to tell Elinor at least two other times, but changed his mind.
His reasons are that he didn't think he had a chance of interfering with success, he thought Marianne's influence might be enough to reform Willoughby and also, as he says, he didn't want to appear to "raise [him]self at the expense of others." Do you think these are good enough reasons?
He must have been very tortured by all of this, knowing what he knew, and yet unwilling to make the woman he loves unhappy--he also took suffering upon himself for the sake of trying to give Eliza I a chance at happiness too. He can't bear the thought of Marianne thinking that he revealed all of this about Willoughby in order to gain on his rival.