Immortality by Dana Schwartz

Hazel Sinnett is alone. She’s half-convinced the events of the year before – the immortality, Beecham’s vial – were a figment of her imagination. She doesn’t even know if Jack is alive or dead. All she can really do is run her free clinic, helping people and maintaining Hawthornden Castle as it starts to decay around her. When saving a life leads to her arrest, Hazel seems doomed to rot in prison until a message intervenes: Hazel has been specifically requested to be the personal physician of Princess Charlotte, the sickly daughter of King George IV. Soon Hazel is dragged into the glamour and romance of a court where everyone has something to hide, especially the ladies of the princess’s close circle, who never seem to stay hurt for long.