Hi Stephanie,
Sorry to hear of all these troubles. As far as phrasing the email is concerned, I wouldn't worry too much. If the grad program director had already been working with you to rearrange your schedule she probably already understands the severity of your situation. Don't worry about whether or not they will get "sick of dealing with you." If they do, then move on. Your life and health are more important than grad school, even though it might not always seem like it.
Be sure to express how thankful you are that they have been accommodating thus far, but say that the severity of your situation would just not allow for you to continue productively. I would approach it less as a "I can't do it" situation and more as a "you deserve the best and full attention from your students" situation. If you make it more about THEM (since they are paying you) I don't think you will have a problem.
"Your school has been so supportive and accommodating, and I thank you for your encouragement. I believe you deserve the best from each of your students and I do not feel I can perform at my peak right now due to these life events. I look forward to coming back and giving this program the attention it deserves."
(That's not great, but... something like that)
Good luck!