How can one purify the heart and dedicate it solely to Allah, and feel true reverence and closeness to Him, after a long period of estrangement and forbidden acts?

Congratulations on what Allah has granted you in terms of commitment to acts of obedience. You should praise Allah and ask Him for more.

To overcome languor (spiritual weakness), you must make sincere repentance, reflect on the texts of encouragement and deterrence, possess strong determination to do good, and avoid all evil. This is because a strong believer in matters of the Hereafter is more beloved to Allah. You should be keen on obeying Allah Almighty, seek His help, and not despair. You must engage in all the rituals of the religion, and your desires should be subservient to the religion.

The rectification of the heart is the origin of all rectification, as the Prophet, peace and blessings be upon him, said: "Indeed, there is a piece of flesh in the body; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Indeed, it is the heart." Therefore, a servant's faith will not be upright until their heart is upright. For this reason, overcoming languor begins with rectifying and purifying the heart, then striving against the self and Satan.