I'm still trying to puzzle out the meaning of these sentences at the end of ch.18:

The compliment of John Thorpe’s affection did not make amends for this thoughtlessness in his sister. [Catherine] was almost as far from believing as from wishing it to be sincere; for she had not forgotten that he could mistake, and his assertion of the offer and of her encouragement convinced her that his mistakes could sometimes be very egregious. In vanity, therefore, she gained but little.