They say, "from the overflow of the heart the mouth speaks" which pretty much means those thoughts, feelings, and emotions deep down in side of you have a way of coming out even if you try to suppress them. Have you ever noticed that if you are in the presence of someone you truly despise you find yourself saying things you wouldn't normally say, or acting in a way you wouldn't normally act. Sure, you might be able to act normally for a short period of time but sooner or later, the more you are around that person, the more evident it is that you do not like them.

In the passages we highlited: (Matt 15:18-20) Jesus talks about this very issue.
He mentions that the evil in our hearts, if allowed to stir there, rises up in thought and affection and flows into voluntary action, that's what really defiles a person.
for out of the heart proceeds evil thoughts. Yep, the heart is at the center of it all pride, envy, greed, materialism, jealousy, and so on. If you continue to ponder on these things and allow them to stir up in your heart and thought life you will act on them.

A prime example would be, if a man continues to have fantasies of cheating on his wife and he allows himself to think often on this theme, creating situations in his mind, pondering and scheming how this can be achieved. No matter how innocent he might think this is, eventually he stands a great chance of weakening and giving in to having an actual affair. He has thought about it for so long, he has justified it in his heart and mind, when the opportunity presents itself, he more than likely will succumb and could possibly lose everything that is dear to him.

What is the cure, what can i do to guard my heart ???

Don't allow elephants in the living room. Seems simple doesn't it. The idea here is that everyone has something in their life that they might be overlooking which is ultimately more important than the things we give our attention to. What do you spend your time doing? What gets your attention? What are your priorities?
Its like the person who has $10 000 dollars of debt on their credit card, they come into a little money and use it to buy a new Playstation or Xbox instead of paying down their debt.

God call us to do more than just be "ceremoniously clean" that's where the Pharisees were going wrong. They went through the motions of being clean but they had an elephant in the living room which had already messed up the whole house.

Think about the things that you dream about, the things that you allow to stir in your heart. ARe they making you unclean?

If there is an elephant in the house, the first thing you need to do is get it out. Get it out and after that you can clean up the mess. But the elephant has to go first

Matthew 15:18-20