A hard heart is one that does not fully recognize our true condition and the danger we are in. The best way to begin change this condition is to remember our eventual fate. Saint Theophan give the following advice:
Say to yourself 'Alas, so will come death.' Another man you know dies; any time it could be your hour. Do not estrange yourself from this hour of death. Convince yourself that the angel of death has already been sent; he is coming, and draws near. Or imagine yourself to be a person who stands with a sword drown over his head , ready to cut it off. Then imagine clearly what will happen at the time of death or after words. James tells us: The judge standeth before the door (James 5:9) At this moment yo will be faced alone with all your sins. There is a choice you are making. It is one between eternal life in paradise or hell. One promises blessedness and the other torment. Will you know God and be accepted or not know Him and be rejected? Theophan says, "Feel all this vividly and force yourself to remain in it until you are filled with fear ind trembling."
Next, he suggest that you turn to God and place yourself before Him. Imagine you sinful nature standing before God and wonder if your presence will be offensive. Will you be able to face Him or will you turn your back on Him?
Next, he suggest that you ascend in thought to Golgotha and crucify yourself. Think about all you have been given as a Christian: redeemed by His blood, cleansed by the Baptismal water, you have been given the gift of the Holy Spirit, you have taken the Blood and Body of Christ at His table,. Imagine being on Golgatha. Will you be one who crucifies Him. Will you repent like the thief? Saint Theophan says. "Choose one or the other: either crucify Him, then perish eternally––or crucify yourself, and inherit eternal life with Him."
Next, he asks you to consider the sin that you hang on to and abhor it and reject it. realize that it is your sinfulness that separates you from God. Saint Theophan says, "It separates you from God, wreaks havoc on your soul and body, torments your conscience, brings upon you God's punishment in life and at death; and after death it sends you to hell, closing paradise to you forever."
Finally, he asks you to look at sin from the devil's point of view. Do you realize that when yo sin you are working for the devil who has done nothing for you? He befriends you through sin and entices you by promising sweetness and then he torments and tortures. Like eve in the Garden of Eden he will try to convince you that your sins are nothing. He is filled with glee when anyone falls in his traps of sin.
"When you will thus press into your heart one after another contrition-producing and softening feelings––little by little your heart will warm up and begin to move, and after it your enfeebled will will begin to strain itself and spring into action. As volts of electricity communicate certain tension and stimulation to the body, or as the cool, clean morning air communicates freshness and energy, so does this feeling that fills the soul awaken slumbering energy and renew the call and willingness to escape your dangerous condition. these will be the beginnings of your active care for you own salvation..."
Ref: Path to Salvation pp 137-139