How can one get rid of their heedlessness, sins, and feeling of despair in Allah’s mercy, and how can they change for the better when they think that Allah is angry with them and will not answer them?

Beware of despair and hopelessness concerning Allah's mercy, no matter how great your sins. Rather, have a good opinion of your Lord, and start a new page with Allah by showing humility to Muslims and rectifying your heart. You will be aided in this by the company of the righteous, persistent remembrance of Allah and supplication, and striving against your soul to rid it of its flaws. Be diligent in reading "Mukhtasar Minhaj al-Qasidin" and "Tahdhib Madarij al-Salikin." Maintain obedience to your Lord and fill your time with beneficial things. Whenever you sin, repent, and do not despair of repentance. For Allah is Oft-Forgiving, Most Merciful, and no sin is too great for Him to forgive. So flee to Him and hope for His goodness and bounty.