How can one deal with an inner struggle stemming from a dark past and ongoing sin, coupled with the fear of falling into disbelief (kufr) after repentance and commitment to acts of worship?

Your feeling of hypocrisy and your desire to return to the past are from the whisperings of Satan. You should thank Allah for the blessing of your repentance from drugs and the sins associated with them. Your feeling of fear of hypocrisy is a sign of faith in your heart, just as the Companions feared hypocrisy. You must abandon the sin you are still committing, perform sincere repentance, and flee to Allah from disobedience, ignorance, and heedlessness. To overcome boredom and weariness, occupy your time with beneficial matters of this world and the hereafter, seek good company, visit graves and the sick, and strive to do good to others.