Congratulations on your piano exam!

I think unaluna has the right idea.

Set a short time limit on how long you will listen to her. When you are five minutes from that limit, say that you will need to hang up in five minutes. When the five minutes are up, kindly say good bye and hang up!

You can also screen her calls and not pick up when you are unable to listen.

In my opinion you must limit her, difficult though she might make that for you, to regain control of your time.

Are you speaking with a therapist?