"if he, who had undoubtedly been always so much the most in love of the two, were to be returning with the same warmth of sentiment which he had taken away, it would be very distressing."

I think, since she had convinced herself that he was in love with her, had discovered that whe was not in love with him & that it would be better to let their "relationship" cool down, it was a relief to her to find that he was not in love with her anymore, so the short visit was forgivable as proof of what she wanted.