What is the way to get rid of the obsessive whispers of returning to misguidance, and how can the heart be firm upon obedience and drawing closer to Allah?

Your fear of backsliding is a healthy thing that motivates you to draw closer to Allah and remain steadfast in acts of obedience. You must have a good opinion of Allah and remember His mercy. Among the things that will help you remain steadfast are: increasing supplication (du'a) and remembrance (dhikr), keeping company with righteous women, contemplating the Day of Judgment and its terrors, and contemplating the Names and Attributes of Allah.