In my theory, her disease is Friedreich's Ataxia. Symptons begins between the ages of 5 to 15. Muscle weakness in the arms and legs and loss of coordination are symptons.
It causes heart disorders. In Episode 20, she may be having a severe and painful tachycardia, and a following cardiac arrest. The surgery may be to implant a pacemaker, but another arrest killed her during the surgery.
My hypothesis is that she is suffering from advanced multiple sclerosis. The extreme exhaustion along with the loss of motor function in her legs (Episode 15-Liar) and hands (Episode 21- Snow) are her most obvious symptoms. However, the risky operation she undergoes is the only thing that make me question my theory.