This is (obviously) not a diagnosis, but it’s possible you don’t have afib. My Apple Watch also reports that I have a percentage of afib each week, although lower than you as I have topped out at 38%. My cardiologist at Mayo Jacksonville has determined I do not have afib, but I do have premature atrial and ventricular contractions, and he believes the Watch is mistaking PACs and PVCs for afib. I haven’t figured out if there is a way to translate my reported afib percentage to PAC or PVC load, so all I’m able to conclude is the lower that number is the better. Anyway, you might want to discuss this possibility with your doctor.