If you couldn’t get a song out of your head and it was causing you distress you would know FOR SURE that the actual song itself was NOT the problem.
If it was just a song you wouldn’t worry so much (though it can be just as wearisome) – you wouldn’t add too many other anxious thoughts or analyse and worry about the lyrics or think you had a problem related to the actual SONG.
You would readily accept that you were suffering from mental fatigue and that it was all tied in with your anxiety. You would easily be able to see that the song was not the problem – the problem is the underlying fatigue and anxiety.
The only reason you can’t see that these anxious thoughts are no different is because you were unaware to begin with that this is a very common problem in a sensitized person and because of your sensitized state you understandably but mistakenly believed that it must all be very serious.
This added to your fear and you got further caught up in the anxiety cycle.