When You Are Down

Why is it that when you are not doing so well, whether it be emotionally, physically, spiritually, or all three, Satan uses this to his advantage. When I am not feeling well, or in pain, I get really down unintentionally. I get extremely unmotivated, and don't want to do anything. I forget the small things of prayer and scripture study because I get lazy, and then I feel even worse. How does Satan do it EVERY TIME!!! I hate it! I just need to get out of this spiritual slump. Any suggestions for how to stay motivated with the small and simple, yet important things when you are down?

 I just need to remember the Savior through thick and thin!