hunger.
I know that might seem un-ordinary to be thankful for. But the very few times I've begun to feel that particular pain-emptiness in my gut, I feel like it's an achievement. At bare minimum, it's secularly a hint that for at least a moment I've successfully gone against the grain of my usual propensity for gluttony.
Spiritually and in mind-soul-identity, I feel true and alive when I'm hungry that I don't when ordinarily full.