STB

No new songs for this week-we talked at length about the problems I'm having with STB.  To my slightly uneducated ear, it sounds uneven and nothing like the actual tune.  I always attributed this to my embellishments being too slow; and yet, when my instructor plays it with really slow embellishments, it still sounds like STB.  This tune is my Achilles heel of tunes.  I know it's the one everyone will want to hear, and it will suck like a Hoover if I never learn to play it competently..  So-my problem according to my instructor is that my tempo is very uneven.  I speed up on some phrases and slow down on others.  I'm going to practice it with a metronome, note by note if necessary.  Instructor says that if I can play Teribus (which I can) then there's nothing in STB that should give me problems.  I know the embellishments; it's a simple 2/4 time; I know the tune by heart and can play the notes in my sleep.  So why does my version sound so awful?