Throughout all the misery with Marianne's situation, Elinor keeps comparing it to what she has experienced with Willoughby. She has good reason to compare. Look at some of the parallels in the situations.

--Following a two (and a little) month separation (beginning of September to mid-November), when Elinor first sees Edward again she is mortified by the coldness and reserve he appears to show towards her.

--Following a two-month separation (November to January) it appears Willoughby has had a change of heart towards Marianne when he first sees her again and absence has weakened his regard for her.

--Elinor learns that Edward has been in the Barton area for two weeks without trying to contact her.

--Marianne is in London for almost two weeks without Willoughy contacting her.

--Edward was involved in an engagement he needed to keep secret

--Elinor learns through Mrs. Jennings' gossip, that Willoughby is engaged and that it has been a secret before now.

--Both Edward and Willoughby had also become very attached to the Dashwood family and treated Marianne and Elinor, respectively, like sisters.

--Both Edward and Willoughby are dependent on an older female relative who is in control of their fortune

I'm sure there are more parllels and similarities. To Elinor, Edward comes up favourably in all comparisons:

Her own situation gained in the comparison; for while she could esteem Edward as much as ever, however they might be divided in future, her mind might be always supported. (ch.28)

and

In her earnest meditations on the contents of the letter, on the depravity of that mind which could dictate it, and, probably, on the very different mind of a very different person, who had no other connection whatever with the affair than what her heart gave him with everything that passed, Elinor forgot the immediate distress of her sister, forgot that she had three letters on her lap yet unread, and so entirely forgot how long she had been in the room

It's true that Edward would never have written her such a horrible letter, and that it does not seem as though he ever wanted to hurt her. I can't see him wanting to publically humiliate her either, although Elinor would also never put herself in the situation Marianne did at the ball. But still--the part about her situation gaining by comparison...

She's still heartbroken in love with a man who is engaged to someone else, just like Marianne. He still, whatever his intent, deceived her as to his availabiliy, just like Willoughby did to Marianne, and he still has never owned up to what he did face to face, just like Willoughby.