"The mere habit of learning to love is the thing; and a teachableness of disposition in a young lady is a great blessing. Has my sister a pleasant mode of instruction?"

I've been thinking about this some more, and could it be that Catherine has indeed learned to love, and it is Henry she has learned to love. She's embarrassed because she's not thinking about Eleanor and hyacinths and her instruction now, but Henry and his.

And this could tie in with Elena's thought about "teaching through pupil's loving bond with a teacher". For Henry has been her teacher in many things, and she has learned to love him.