I'm going to break a pinky promise with a girl that we would dance together at prom.
What would prompt you to reneg on your promise to dance with a girl?
She betrayed me by dancing with another guy on the school formal night.
I understand that you may feel betrayed by this girl dancing with someone else, but it is not responsible for you to break your promise to dance with her. Breaking promises shows that you are untrustworthy.
I'm still upset. I'm sorry but I'm going to break the promise.
You are going to really disappoint her. Just because she danced with someone else doesn't mean that she doesn't like you.