In the episode Coda, why did Kevin lose confidence in playing the piano at the end? What was his fear?

I think it's pretty self-explanatory in the episode. The piano teacher allocated the same song for Kevin to play at the recital that the other more experienced kid was going to play.

Kevin's fear of being shown up as "not as good" as the other kid led to nerves and errors during his piano performance.

Yeah I have not seen the episode since 2005 over 20 years ago the adult female teacher is a bad piano instructor she had kevin play the same song for weeks kevin had the confidence and determination and was prepared she should had him play multiple songs just in case he had some competition