Elizabeth, I'm not sure of it's cancer related, thyroid, depression,medications or something else. But given the way the health care system is these days you really have to advocate for yourself. Unfortunately, it may require going to several different doctors till you find the answer to what is causing the fatigue. And, of course when you are tired who feels like running around to all these different doctors.
As the mom of 4 kids,I know there were days when my kids were young that I thought exhaustion was a way of life. My kids are great but they take up a lot of energy. I hope you can get some breaks for yourself(I know that's tough) till you solve this.