How can one soften the heart and repent when there is no desire to do so, given the heart's hardness, abandonment of prayer, and commission of sins?

Abandoning prayer is, by consensus, one of the most heinous sins, greater than fornication, theft, drinking alcohol, and killing a soul. Therefore, whoever abandons prayer must immediately repent, maintain their prayers, and abandon sins. Among what helps with this are: frequently contemplating death and its terrors, visiting graves, visiting the sick, and being mindful that one's term may end at any moment. Also, contemplating Allah's names, attributes, power, and His constant watchfulness, recognizing the effects and harms of sins, associating with righteous and virtuous people, consistently engaging in dhikr (remembrance of Allah) and sincere supplication, and striving against the self. Allah says: "And those who strive for Us – We will surely guide them to Our ways." (Quran 29:69)