Too much power over her heart had already been acquired.

The ideal of the man had grown too suddenly into a most palpable image of beauty and perfection.

Earnestly did her heart plead for him.
Sad, even to tears, was it, at the bare thought of giving him up.
There was yet burning on her pure forehead the hot kiss he had left there a few hours before--her hand still felt his thrilling touch--his words of love were in her ears--she still heard the impassioned tones in which he had uttered his parting "God bless you!" Thus it was with the gentle-hearted girl, exposed, far too soon in life, to influences which stronger spirits than hers could hardly have resisted.
Midnight found Mrs.Markland wakeful and thoughtful.

She had observed something unusual about Fanny, and noted the fact of her early retirement, that evening, from the family.

Naturally enough, she connected this change in her daughter's mind with the letter received from Mr.Lyon, and it showed her but too plainly that the stranger's image was fixing itself surely in the young girl's heart.
This conviction gave her pain rather than pleasure.