be open and honest and explain everyting truthfully
Be honest. Say you want to move somewhere else, where taking lessons is easier for you and that your intention is to be back at their barn in the future.
Tell her the theory say look I love you as an instictor but I just can't afford these lessons anymore so I am going to move to.....
I think that being honest is the best policy :) hope this helps. Been there done that with instructors it felt awful but in the end it's how you feel with your progress and your the one paying for them so you get to choose where you go and whom you have lessons with.