The Last Waltz by Mary Balogh

Gerard Percy, the new Earl of Wanstead, dreads returning home to Thornwood, where the widow of his predecessor lives with her two children. He has unhappy memories of Christina, the widowed countess. But it is Christmas, and he decides to take a houseful of guests with him to make his return easier.

For her part, Christina feels an equal dread of seeing Gerard again, ten years after she renounced him in order to marry his strict, puritanical—but seemingly safe—cousin. She is horrified when she learns about the guests and the fact that Christmas this year is to be a bright and merry occasion, for her late husband would allow no frivolity of any kind in their life.

And so they meet again, these two former lovers who have bitterly resented each other for ten years—but never forgotten each other.