In spite of his failures and falls, I believe David's secret lay in the fact that he understood God, who "searcheth all hearts" and his heart's cry was, 'My heart is fixed. Oh! God, my heart is fixed.' In an old phrase David "knew his heart's home."
There is a difference between losing ones way and losing ones destination. One may find himself temporarily thwarted by the enemy for one reason or another but God's faithfulness will bring the soul which longs for Him to ultimate triumph - that which is His will not be taken from His hands.
We live in a day of 'positive teaching;' of emphasis on the mechanics of faith and of following God. Sometimes it can be devastating to realize that with all our understanding we are so exposed to failure when we try to do it together and make it all work.
Paul's cry, "when I am weak then am I strong," will be our cry also when we realize that God takes full responsibility for the hearts and lives which are given Him. - And HE, not our wishful dreaming - will give us the desires of our hearts.
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Rejoice not against me, O mine enemy: when I fall, I shall arise; when I sit in darkness, the LORD shall be a light unto me. Micah 7:8
O God, my heart is fixed; I will sing and give praise, even with my glory.Psalm 108:1
My heart is fixed, O God, my heart is fixed: I will sing and give praise.Psalm 57:7