A letter of recommendation?

I forgot the title of the book, but I still remember the content. At the beginning of the novel, it was said that the female protagonist was a personal servant girl who lent her belly to give birth to a child for the young lady she served. Once the child was born, the young lady who had been very good to her actually poisoned her when she was weak. The female protagonist was poisoned and reborn as a young lady of an official family. The male protagonist was reborn in a general's family after an accident in the modern world.(It seems to be, otherwise, it's also a high-ranking official's family). The male and female protagonists were reborn into these two children at the same time. These two children were cousins or something. Moreover, it was those two children who fell into the water while playing together that gave the male and female protagonists the chance to be reborn into their bodies. The male lead had always thought that the female lead had also transmigrated and was reborn, but he did not want to tell him. In fact, the female lead had only been reborn in her original living environment. The female protagonist had also decided to change completely because she had the chance to be reborn, while the male protagonist had always regarded the female protagonist as his fellow countryman (because he believed that the female protagonist had also transmigrated and been reborn) and had been chasing her. In the end, the male and female protagonists got together. As the male protagonist had come here now, he had been taught to be monogamy. He swore to marry only the female protagonist. After many twists and turns, the two of them finally got together. The female protagonist had also let go of her previous hatred. That's about it. I still remember that the novel's description of the female protagonist's gradual growth is relatively large.