Proverbs 17:20 says, "He who has a deceitful heart finds no good, and he who has a perverse tongue falls into evil."
We know from Jeremiah 17:9 that, “The heart is deceitful above all things, And desperately wicked; Who can know it?"
So most everyone has a deceitful heart that we can not trust. But 1 Samuel 16:7 tells us that God looks on the heart and not on outward appearance, so if everyone'sheart is wicked how could God look at David's heart and choose him to be anointed as Israel's king?
God Himself testified of David, "I have found David the son of Jesse, a man after My own heart, who will do all My will." So David was different because he didn't listen to his heart, he understood his heart was wrong and he wanted to have a heart just like God's heart. David said in Psalm 51:10 "Create in me a clean heart, O God; and renew a right spirit within me."
As we look back to Proverbs 17:20 James 3:8 says, "But no man can tame the tongue. It is an unruly evil, full of deadly poison."
If no man can tame the tongue, then aren't we doomed to fall into evil no matter what? Won't we just be continually defiled(Matthew 15:11)? And how can we be doers of the Word not hearers only (James 1:22) if we can't control ourselves?
David says in Psalm 141:3, "Set a guard, O Lord, over my mouth; Keep watch over the door of my lips." (that entire Psalm is a good one to read on the topic of hearts and tongues really) David went to God for help because he could not help himself.
Hebrews 4:12 says, "For the word of God is living and powerful, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart."
So if you want to be a better person the only way is through heart surgery (Ezekiel 36:26) and the surgeon is God Almighty! You can't change yourself so you need God to change you. That's why God sent His Son Jesus Christ to die for all your sins on the cross, if you repent and make Jesus Christ your Lord you will be forgiven and free to be the man or woman that only God could make you into!
Jesus Himself paralleled our Proverb in Luke 6:45, "A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh."
If you want to control your tongue you have to control your heart and the only way this work is accomplished is through reliance on the one true God!