Willpower is a limited resource. Science proves this. While you can try to waste less on little things, or build it up with other tricks, it’s never going to be infinite. And chronic illness demands a lot of it; you have to do things that are when you’re in pain, or exhausted. Maybe you need to stick to rigid treatment and lifestyle plans. Many things will be uncomfortable.
If you just can’t find the motivation to do everything you are asking of yourself, it’s probably not because you’re “lazy.” It’s not because you’re a bad person. It’s because you’re burning through your willpower too fast. When I start struggling to stick to my daily routine, I know I’ve overdone it and I need to scale back. And you know what, my self control goes back to normal after a rest. I am very confident it is a problem with my situation, not my personality.