How can one atone for the sins mentioned, especially with a hardened heart, the loss of devotion in prayer, and the repeated vows not to return to sins without fulfilling them? And was the inability to perform Hajj due to sins?

It is obligatory upon you to hasten to sincere repentance, to observe the prayers, to sever all forbidden relationships, and to seek forgiveness from those whose spouses you corrupted. What will help you with repentance is reflecting on the Day of Judgment and the Fire, considering the consequences of sins, associating with the righteous, adhering to remembrance [dhikr], persisting in supplication [du'a], and striving against your lower self [jihad al-nafs].