I just realized that mindfulness meditation is all about nonjudgmental acceptance of the experience of failing at mindfulness meditation. Mindfulness meditation always involves failure, therefore it requires nonjudgmental acceptance.
We fail at mindfulness because we are always more or less distracted; we cannot achieve perfect mindfulness.
Practice does not make perfect; practice makes practice.
If the goal of mindfulness is to practice, then we cannot fail, unless we fail to practice.
But if we practice nonjudgmental acceptance, failing to practice is neither failure nor victory.
All this brings us back around the circle to never failing, but just being.