this particular sentence in CH 37; (Yes, I'm lagging a bit, but have finally caught up :-)

When Elinor narrates the incidence at Harley Street and tells her about Edward's engagement, Marianne is going through all kinds of thoughts, and feelings.

"But Marianne for some time would give credit to neither. Edward seemed a second Willoughby; and acknowledging as Elinor did, that she had loved him most sincerely, could she feel less than herself! "

First there is the part - and acknowledging as Elinor did, that she had loved him most sincerely,

This comes straight after mentioning Willoughby, so is she talking about herself and her love for Willoughby or Elinor's love for Edward, because it is followed by the part - could she feel less than herself!

I also don't understand what 'feeling less than herself' could mean.