the spirit is willing, but the/(one's) body is weak

One has the desire or intention to change do something beneficial, but one lacks the resolve or motivation to do it. Every year I resolve to eat better and do more exercise; and every year, inevitably, the spirit is willing, but the body is weak. We all know someone who has the best intentions in the world to get their lives on the right track, whose spirit is willing, but their body is weak.

the spirit is willing, but the/(one's) flesh is weak

One has the desire or intention to change do something beneficial, but one lacks the resolve or motivation to do it. Every year I resolve to eat better and do more exercise; and every year, inevitably, the spirit is willing, but the flesh is weak. We all know someone who has the best intentions in the world to get their lives on the right track, whose spirit is willing, but their flesh is weak.

the spirit is willing (but the flesh is weak)

someone has good intentions (but yields to temptation and fails to live up to them).

This expression quotes Jesus's words in Matthew 26:41, on finding his disciples asleep in the Garden of Gethsemane despite his instructions that they should stay awake.