Looking for a specific book where Darcy is Sick

I read a good variation a few years ago and am having trouble finding it again:
It opens with Darcy proposing because he thinks he is dying and needs an heir. Elizabeth marries him and does a bunch of research to try to help alleviate his symptoms, which lead to him starting to get better they are doing well until he goes to Hunsford without her...

Can anyone help? TIA