The Wood Nymph by Mary Balogh

Lady Helen Wade is not interested in living the proper life of a proper young lady. She prefers to slip away to her own place in the woods, where she can wear comfortable clothes and let her hair down and go barefoot while she dreams and paints and communes with nature. There she meets the rich and handsome William Mainwaring, newly arrived owner of the property upon which the woods stand.

William, nursing a broken heart, is enchanted by the simple country girl whom he knows only as Nell, and returns a number of times in the hope of seeing her again. A brief, passionate affair ensues before guilt sends William abruptly away so that he can avoid further temptation to ruin her.

When they meet again, Helen and William are both in London, and he knows her for who she is. How can he persuade her that he is not the heartless cad she believes him to be when he knows he behaved badly? And how can he persuade her that he loves both Nell and Helen—and that somehow he is worthy of her love?