Austen Said:

as the ruin of all his happiness in Fanny,

had there been no young woman in question, had there been no young person of either sex belonging to him, to be endangered by the society or hurt by the character of Mrs. Rushworth, he would never have offered so great an insult to the neighbourhood as to expect it to notice her. As a daughter, he hoped a penitent one, she should be protected by him, and secured in every comfort, and supported by every encouragement to do right, which their relative situations admitted; but farther than that he could not go. Maria had destroyed her own character, and he would not, by a vain attempt to restore what never could be restored, by affording his sanction to vice, or in seeking to lessen its disgrace, be anywise accessory to introducing such misery in another man's family as he had known himself.

it was anger on Fanny's account; he must get the better of it, and make Mrs. Rushworth Maria Bertram again in her treatment of himself.

how impossible it was that he should ever meet with such another woman,

whether a very different kind of woman might not do just as well, or a great deal better: whether Fanny herself were not growing as dear, as important to him in all her smiles and all her ways, as Mary Crawford had ever been; and whether it might not be a possible, an hopeful undertaking to persuade her that her warm and sisterly regard for him would be foundation enough for wedded love.

She was of course only too good for him;