Does linking happiness to worldly pleasures and the delayed expansion of the heart after sincere repentance indicate insincerity in repentance, or is continued stumbling, despite attempts to return to Allah, normal in this situation?

The mercy of Allah encompasses all things, and the door of His grace and generosity is not closed in the face of anyone, no matter how much a servant has wronged himself. Have a good opinion of Allah, turn to Him with sincerity and truthfulness, strive in your supplications, and exert your utmost in seeking His pleasure. Do not let Satan deter you from repentance with these false claims; rather, repent to Allah immediately. Do not despair of the spirit of Allah, nor give up hope in His mercy, and supplicate to Him for whatever good you desire in this world and the Hereafter. Your fear of Allah and your apprehension concerning your sins is a good thing, but this fear should not lead you to despair of Allah's mercy; rather, couple it with hope in His grace and mercy. Abandon sins and attend to your affairs, and busy yourself with the obedience of your Lord, sincerely devoted to Him, having a good opinion of Him, and trust that He will not disappoint you nor forsake you.