"What you do every day matters more than what you do once in a while."
It's a pointed observation by Dr. Samuel Johnson, in The Rambler issue #28 (June 1750):
One sophism by which men persuade themselves that they have those virtues which they really want, is formed by the substitution of single acts for habits. A miser who once relieved a friend from the danger of a prison, suffers his imagination to dwell for ever upon his own heroic generosity; he yields his heart up to indignation at those who are blind to merit, or insensible to misery, and who can please themselves with the enjoyment of that wealth, which they never permit others to partake. From any censures of the world, or reproaches of his conscience, he has an appeal to action and to knowledge; and though his whole life is a course of rapacity and avarice, he concludes himself to be tender and liberal, because he has once performed an act of liberality and tenderness.
As a glass which magnifies objects by the approach of one eye to the lens, lessens them by the application of the other, so vices are extenuated by the inversion of that fallacy, by which virtues are augmented. Those faults which we cannot conceal from our own notice, are considered, however frequent, not as habitual corruptions, or settled practices, but as casual failures, and single lapses. ...