The Constant Heart by Mary Balogh

Rebecca Shaw was heartbroken when Christopher Sinclair, whom she had loved passionately and who had sworn that he loved her in return her, left her without explanation to marry a far wealthier woman. Rebecca has since found consolation and peace as the fiance of the local vicar, whose worthy work and dreams she shares. But Christopher, now widowed, is back and Rebecca has to keep reminding herself that he once betrayed her and must never be trusted again. Her heart, however, tells her otherwise.