What is the solution for a person aged 20 who feels that someone is watching him when he leaves the house, even though he does not show fear of it?

The feeling of being watched may be a satanic whisper (waswasah), and to get rid of it, you must remember Allah, rely on Him, and trust Him. For with the remembrance of Allah do hearts find peace, and whoever believes in Allah, He guides his heart, and whoever relies on Him, He is sufficient for him. Know that what has befallen you was not going to miss you, and what has missed you was not going to befall you. Calamities and death are in the hands of Allah alone, and nothing will befall you except what Allah has decreed. Belief in divine decree (qadar), both its good and bad, is essential. For if you were to spend gold the size of Mount Uhud in the way of Allah, it would not be accepted from you until you believe in the entire divine decree, and that what has befallen you was not going to miss you, and what has missed you was not going to befall you, and if you die upon other than this, you will enter the Fire.