] "Elinor now found the difference between the expectation of an unpleasant event, however certain the mind may be told to consider it, and certainty itself. She now found, that in spite of herself, she had always admitted a hope, while Edward remained single, that something would occur to prevent his marrying Lucy; (Snip)

] While she certainly isn't in hysterics, like Marianne, Elinor's good sense, (as we have seen it), should have prepared her for the event better. The veneer of her sensibility has been cracked, and then, when she learns Edward is not married afterall, she completely looses the self command that had been her dominant trait for the bulk of the novel.

Yes, but bear Elinor's circumstances in mind. She's isolated, waiting for Colonel Brandon to visit as he will have news of Delaford. She's subject in her imagination to Lucy as Edward's wife (notice she doesn't wish ill on Lucy Steele,she merely wishes a better marriage prospect on her). She attempts to steel herself to hear the news of Edward's nuptials;while both dreading and expecting it. Then the gent on horse turns out to be Edward-shock! Then as his news must have appeared inexplicable to Elinor she became confused in her sensibilities. I doubt she's had an attachment before and has had little experience of men romantically. Like everyone,Elinor has both reason and emotion. When she becalms herself,her sense will surely return.Let's take that into account too.