As far as I can tell (and please do correct me, anyone and everyone, if I'm wrong), unlike in NA, where Austen has the narrator use the "I" with some frequency, this is the only time she does so in S&S. If indeed there is only this one use (rather than my goof in missing others), I wonder if it is coincidental, or if there is a particular significance to Fanny's inner world, revealed in this passage, that merits special, intentional treatment of this sort? Austen does not employ this device when relating the inner thoughts of other characters in the novel.