People often try to fill their hearts with the danger of what they are doing. You can tell yourself ‘if I keep doing this it will cause problems for me.’ That may be true and could be good ‘smelling salts’ to get you to recognize your problem. But if that is all you say to your heart, it (as it were) bends the metal of your heart but doesn’t really soften and permanently reshape it. The motivation is ultimately selfish and it only brings short-term change.
We need to go deeper to the only lasting way to change our hearts — take them to the radical, costly grace of God in Christ on the cross. You show your heart the infinite depths to which he went so that you would be free from sin and its condemnation. This fills you with a sense not just of the danger or sin, but also of its grievousness. Think about how ungrateful it is, think of how your sin is not just against God’s law but also against his heart. Melt your heart with the knowledge of what he’s done for you. Tremble before the knowledge of what he is worth — he is worthy of all glory.