You go and talk to your boss in person in private - face to face.
I'm sorry, Mr. White, but I am unable to continue to working in my current job. I've had some medical issues come up that has forced me to take a step back from working.
I appreciate all of the training that I've had. It's been a good company to work for so far, but at the present time, I must focus on my health. Thank you so much.
Shake his or her hand.