"Lean not on your own understanding"
It means that we establish ourselves in his word (Bible)
The heart will confirm what the mind is trying to perceive.
David is speaking to his inner parts not allowing them (the desires and wants and lusts) to dictate how his body responds.
All my sins.....My what a glorious thought that when I confess and repent and turn from my sins, God forgives everyone of them.
Read it slowly and put yourself in the verses.
Everywhere you see the word me, I or mine...insert your own name and see the precious promises that God has spoken.