We fall back again and again into our old bad habits because of our weakness on all levels: the flesh, the heart, the will, and the intellect. Our minds are dark, our wills are weak, our hearts are foul. How do we know this? By honest self-examination and a refusal to evade the truth.
The will is not strong enough to tame the animal in us and control its natural tendencies; but it is strong enough to suborn the intellect and persuade it to rationalize the free will's wrong decisions.
A will too weak to tame the flesh is yet strong enough to suborn the intellect.
Because we cannot significantly improve ourselves by our own efforts, we must seek help elsewhere, but obviously not from those who are as wretched as we are, which is to say, from fellow human beings.