If I were the parent, I'd say

 

"I love you.  Your happiness is my happiness.  You are still a minor.  I am responsible for your wellbeing.  I want to help you make the right decisions by thinking it through.

"Everyone has responsibilities to oneself.  You may not like the burden of being responsible, and you want to change things so that you don't have to be burdened.

"By focusing on the change, you have relieved yourself from being responsible for a lot of things, one of which is being your true self and the enjoyment of being a teenager.

"You are running away from your true self and running toward a persona.  That persona entails responsibilities, too.  When you get there, you may be disappointed because it is responsibility that you are running away from.

"I want to help you sort out all your fears and have a clear rational mind to deal with the difficulties in your life so that you grow up happy and strong."

 

The above is an outline of general reasoning, may be used in other kinds of situation as well.  Use your own words to make it more relavent.

Disclaimer: not a professional advice.