How can one regain the pleasure of worship and intimate communion with Allah after a period of spiritual listlessness, intrusive thoughts, and justifying sins, especially after having previously experienced the sweetness of faith?

Your heart feeling deficient is a sign of good. Look for the reasons behind the change in your state; it could be a sin or being too hard on yourself. Address these reasons, then turn to yourself to rectify them, and you will return to your vigor and eagerness in worship.