Stable systems return to their starting position after a nudge, unstable systems increase the effect of the nudge.
A ball in a hole is stable, a person on a tightrope is unstable.
I learned this concept in mathematics, but I can apply it to habits.
If I'm using willpower to force myself to eat healthy food, one slip would remind me how much easier it is to eat food I enjoy. Using willpower over the long term is unstable.
If I find a way to associate exercise with feeling good, then even when I don't exercise for a while I don't hesitate to start again. Making it easier to want to do a habit is stable.
What systems are you using that are unstable?