Sentence structure and meaning

[These powers received due admiration from Catherine, to whom they were entirely new; and the respect which they naturally inspired might have been too great for familiarity, had not the easy gaiety of Miss Thorpe’s manners, and her frequent expressions of delight on this acquaintance with her, softened down every feeling of awe, and left nothing but tender affection. ]

This is an excerption from the novel of ' Nothanger Abbey' and, as for me, very difficult to understand- which word or grammatical elements might be connected with the word ' had not'? . Could you expadiate the meaning or grammatical role in this word ' had not' ?