How can one mend oneself if one feels despairing of Allah's guidance, has lost the sweetness of faith, no longer denounces evil, becomes lazy in acts of worship, and whose heart has become like a rock towards the Quran, yet is moved by singing, despite their continuous prayers for guidance?

Your feeling the pain of sin and the bitterness of being far from Allah indicates that there is good within you. Invest this goodness by turning to Allah sincerely, and supplicate to Him with devotion, for Allah does not reject a repentant person and answers the distressed. Seek help from the company of righteous people and strive against your soul. Do not despair if you fall into sin; rather, hasten to sincere repentance and repeat it no matter how often the sin recurs, and do not despair of Allah’s mercy. Learn Islamic knowledge, observe the obligatory acts and increase in supererogatory ones, reflect on Allah’s Most Beautiful Names and Attributes and on the Hereafter, read the Quran with contemplation, and listen to beneficial lectures. Beware of despairing of Allah’s mercy.