Robin by Frances Hodgson Burnett

The other woman who loved and was loved by him moved about her world in these days with a face less radiant than the one people turned to look at in the street or in its passing through the house in Eaton Square. Helen Muir's eyes were grave and pondered. She had always known of the sometime coming of the hour in which would rise the shadow -- to him a cloud of rapture -- which must obscure the old clearness of vision which had existed between them. She had been too well balanced of brain to allow herself to make a tragedy of it or softly to sentimentalise of loss. It was mere living nature that it should be so. He would be as always, a beloved wonder of dearness and beauty when his hour came and she would look on and watch and be so cleverly silent and delicately detached from his shy, aloof young moods, his funny, dear involuntary secrets and reserves. But at any moment -- day or night -- at any elate emotional moment _ready_!

She had the rare accomplishment of a perfect knowledge of _how to wait_, and to wait -- if necessary -- long. When the first golden down had shown itself on his cheek and lip she had not noticed it too much and when his golden soprano voice began to change to a deeper note and annoyed him with its uncertainties she had spared him awkwardness by making him feel the transition a casual natural thing, instead of a personal and characteristic weakness. She had loved every stage of innocence and ignorance and adorable silliness he had passed through and he had grown closer to her through the medium of each, because nothing in life was so clear as her lovely wiseness and fine perceptive entirety of sympathy and poise.