Man and Maid by Edith Nesbit

Maurice Brent knew a great deal about the Greek anthology, and very little about women. No one but himself had any idea how much he knew of the one, and no one had less idea than himself how little he knew of the other. So that when, a stranger and a pilgrim hopelessly astray amid a smart house-party, he began to fall in love with Camilla, it seemed to be no one's business to tell him, what everybody else knew, that Camilla had contracted the habit of becoming engaged at least once a year. Of course this always happened in the country, because it was there that Camilla was most bored. No other eligible young man happened to be free at the moment: Camilla never engaged herself to ineligibles. The habit of years is not easily broken: Camilla became engaged to Maurice, and, for the six months of the engagement, he lived in Paradise. A fool's Paradise, if you like, but Paradise all the same.

About Easter time Camilla told him, very nicely and kindly, that she had mistaken her own heart -- she hoped he would not let it make him very unhappy. She would always wish him the best of good fortune, and doubtless he would find it in the affection of some other girl much nicer and more worthy of him than his sincere friend Camilla. Camilla was right -- no one could have been less worthy of him than she: but after all it was Camilla he thought that he loved, Camilla he felt that he wanted, not any other girl at all, no matter how nice or how worthy.