It is with great regret that I write to inform you that I can no longer meet the terms of my contract and must offer my resignation.
My daughter has been diagnosed with a chronic autoimmune disease that will require me to accompany her to numerous medical appointments on an ongoing basis. Additionally, I will need to oversee her transition back to school. She will require injections multiple times per day along with constant monitoring. Her doctors assure me that after an adjustment period, her care will seem routine (though it's hard to imagine that now!), and they anticipate that as she gets older she'll be able to self-manage her medical and nutritional needs more and more.
Were it not for this personal development, I would never have considered leaving XYZ company. I love it here, and I feel I've contributed a lot to the organization during the past four years.
While our contract indicates that I must resign if I become unable to work a traditional 8 to 5 schedule in the Anytown office location, I would very much be open to staying if a more flexible schedule could be accommodated. I could use the weekends and evenings, or work from home, to make up for missed time due to doctors appointments and school commitments. I would also be happy to train a replacement, work fewer hours, or job-share, as long as I'm able to leave when a medical issue arises.
Please don't hesitate to let me know if a new situation can be hammered out. I assure you I am as dedicated to XYZ's values as ever. If the contract cannot be renegotiated, I appreciate the opportunity to have worked with you, and resign effective immediately.