About Living Wills

In the process of bouncing in and out of emergency rooms the past few months, I quite abruptly found myself with a new thought about my living will: specifically, that even though God often gives us great opportunity to live and thrive; parents, relatives, bodies that can often cure themselves, for each of us the time will come when we can no longer support ourselves, and eventually even the limits of science cannot restore or save us. I respect and admire the science that enables us to remain alive beyond our ability to support ourselves, but I wonder what is God's Will for us. My new thought is this: when we reach the point we can no longer support ourselves, particularly when continuing to live would involve great pain and expense, would it be sinful (against God's Will) to disallow continuing medical support? In other words, could science be keeping us alive beyond God's Will? Comments and reference to pertinent documents would be very helpful. I have the Catholic Three Beliefs brochure don't sense an answer.