We have none of Lady Mary's letters to Hervey, because his son returned them to her after his death, causing her to say that Hervey was the only man she could trust everything with and her bff forever. And then the letters were destroyed, either by Lady Mary herself or by her daughter after her death. Anyway, this is why we're in luck Algarotti, being not an English gentleman, did not return Lady Mary's letters to her but evidently kept them for Byron to find in Venice decades later.