He could say no more, and rising hastily walked for a few minutes about the room. Elinor, affected by his relation, and still more by his distress, could not speak. He saw her concern, and coming to her, took her hand, pressed it, and kissed it with grateful respect. A few minutes more of silent exertion enabled him to proceed with composure. (Chapter 31)

I often feel Brandon’s reasons for keeping silent so long are insufficient, from my modern point of view or perhaps more accurately my romantic point of view (giggle) it seems saving Marianne from Willoughby should have overridden other concerns. Undoubtedly, however, my point of view is somewhat skewered in favor of his steeling the hero role from Willoughby and should not take away from Brandon’s more thoughtful and generous considerations which do him a credit IMO. He worries that exposing Willoughby will hurt Marianne for no reason as Willoughby may be influenced by her to be a better person; it is also unlikely that Marianne would believe Brandon as even at the end of Chapter 31, after she receives his horrible letter in Chapter 29 she still desires to acquit Willoughby of wrong doing. I think Brandon does see truly into Marianne’s heart and his worry that she would see him as jealous is valid; IMO she would have seen his story as trying to raise himself in her esteem at the expense of Willoughby because she is so prejudiced in Willoughby’s favor. IMO It is one of those ironic setups that JA loves—Brandon stays quiet about Willoughby because he does not want to be seen as manipulative, raising himself by disparaging Willoughby but Willoughby is just the opposite—in Chapter 10, Willoughby disparages Brandon openly and is obviously admired by Marianne.