I thought Mr Collins so resigned to Lizzy's refusual as her own mother bursts out "She is a very headstrong foolish girl." He immediately seizes on these words as he feels such defects in her nature could hardly contribute to his matrimonial happiness. And needless to say,Elizabeth as his wife could offend his noble patroness. So it's not difficult for Mr Collins to feel resigned. 'His regard for her was quite imaginery' anyway according to the text. Rather than think less of Lizzy personally, he now regards her as irrelevant as a companion for his future happiness.