IMO, the most telling part of Marianne's long speech regarding her own failings is this: "To John, to Fanny, -- yes, even to them, little as they deserve, I had given less than their due."

To me, this shows that Marianne's change is not just an over-reaction. She has put real thought into her past behavior, and with discernment, is spelling out all the things for which she is to blame. I say with discernment because she's aware that John and Fanny deserve little respect, yet she failed to even give that little. Had this been a self-flaggelation where she blames herself for everything without taking into account the nature of the undeserving, then I would say that it was a thoughtless repudiation of her former self.

It takes a greatness of mind, I think, to realize on the one hand, that John and Fanny are not worthy of her devotion and love, yet they do deserve some, (if only very little), respect.