I think you're doing fine with the exercise. It's just illustrating something to you about the way your mind works.

There is a popular saying in metaphysical / success circles..."What you resist, persists"

In other words, no matter how hard you try to "battle an enemy to the ground", the battle is self-defeating because your attention is always on the "enemy" you are battling, which just gives that "enemy" more power to battle you with in return.

Perhaps instead of trying so hard to get the songs out of your head, just make it okay that they are there. Try acknowledging the presence of them and don't make it a big deal if they stay in your mind. Just let them be. With repeated practice and a gradual willingness not to resist, you'll master this exercise.

I think part of the reason for that exercise is to illustrate to the reader just how little most of us are really in control of our thoughts even though we might like to kid ourselves that the opposite is true.

I remember from studying the book (some years ago now) that there's a great quote in there somewhere later about it being easier for someone to control a kingdom than to control themselves :)

I think the realization you can eventually come to by persisting with this releasing-of-unwelcome-thoughts exercise is that the elegant way to overcome something you don't want is not to focus upon it but just to "let it be" and focus elsewhere instead.

As a general life principle, I think that's a good way to live :)