There are so many words for heart in the Bible. psychē, Greek for heart (used 550 times in the Old Testament, 90 times in Proverbs alone) basically means, "inner man, seat of appetites, emotions, passions, courage and will."
Our hearts seem pretty important. So important, in fact, that the Bible says in Proverbs 4:23 to,
"above all else, guard your heart, for it is the wellspring of life."
The New King James Version says
"Keep your heart with all diligence; For out of it [comes] the issues of life."
The word for diligence literally means to keep in jail or prison.
If all issues in our lives come from the wellspring of our hearts, it seems imperative that we guard them. But, what will we protect by keeping our hearts in prison? In Matthew 12, Jesus rips the Pharisees by saying, "You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks. . .But I tell you that men will have to give an account on the day of judgement for every careless word they have spoken. For by your words you will be acquitted, and by your words you will be condemned." Ouch. Think about some of the simply careless things you've said. I don't know about you, but I don't even have to try to remember. Even the silly, careless things, that really aren't deliberate condemn me. Out of the overflow of the heart the mouth speaks. So, really, those careless words can be traced back to the heart. Living a life of transparency is harder than I thought.
We have hope though. We do have a redemption through Jesus Christ. Jesus Christ came to take away all our sins, including the evil in our hearts. And He wants to start now! Give Him the reins and let Him dig out the evil in your heart. From experience, I know it hurts, but it's so worth it. The pain of selflessness won't go away, but it will lessen as we go through (what I like to call) daily heart cleaning. So many times in the Bible it talks about how even if we think an evil thing or lust in our hearts for something we don't have, we are sinning. The root of murder in anger, the root of adultery in lust, the root of coveting is by itself a heart issue. It all goes back to our hearts. They are the wellspring of life and if we don't protect them, we can hurt ourselves and the people around us.
How can we protect our hearts? That's up to you and God. It's a personal conviction that is between us and our Creator. Oh, yes, you may not murder, but are you angry at someone? You may not commit adultery, but do you lust at one another? You may not lie, but do you wish you could? It's all a heart issue, and it's something that we all have to deal with. Once we get our hearts under control, God can use our actions for good. I encourage you to take a look at your heart today. Are the thoughts you have something you'd be ashamed to say out loud? Pretty soon, as the Bible says, it's going to overflow out of our hearts and fill our mouths. God will forgive us of all our sins, even the ones we think about, but wouldn't it be nice to avoid some of them altogether?
The next step to controlling what our hearts put out is reading God's word. When we fill our hearts and minds with good things, good things can't help but come out of us.
"For the word of God is living and active. Sharper than any double edge sword, it penetrates even to dividing the soul and spirit, joints and marrow; it judges the thoughts of the heart." Hebrews 4:12
Don't let your heart control you, use God's word to judge your thoughts and just watch what God will be able to do through you.
Strive for a life of holiness and practice guarding your heart. You don't have to go it alone either. Find some grounded friends who can keep you accountable. My friends have helped me to get through some of my hardest temptations. And don't forget that God is on your side.