Another 'help me find...'

I read this a couple of years ago and cannot remember for the life of me what book it is.

A woman goes to see her doctor and a few days later receives a letter saying she is very ill and only has a bit of time left. Book is, I think, about making the most of that time, and how she deals with thinking her death is imminent. Turns out, at the end of the book, that the doctor had seen two patients that day that he needed to write to, and the letters got muddled. There was nothing wrong with her after all...