What Makes a Writer Keep Going?

I have had a discouraging day. Nothing in particular happened I just felt drained and unmotivated. I tried to write on a historical YA novel I am working on but only got two pages done. I did edit the next chapter in JESSIE and applied for an online tutoring job but still felt discouraged. I am still in pain from the back injury and am trying to keep a positive attitude but it's almost been a month now and although I am better overall I still have pain and cannot do much without paying for it. I feel like I am spinning my wheels or rather my wheels are stuck in mud and I can't drive out of the mud. Part of the problem is that I am not sleeping well due to pain at night. Sometimes I get up at 3 or 4 in the morning and take pain meds and read until I feel sleepy again. Last night I woke at four, read a bit, then woke up at six. I tried to go back to sleep but had to get up and take pain pills. By then I was awake and the cats were too. I hope that tomorrow I will be able to concentrate more and get some writing done. I looked at AINE on Kindle and think I might put another up when I get another one done that is!!!!