I strongly dislike the word "fire" in relation to changing sponsors or letting a sponsee go. It implies some sort of contractual control of one person over another.
I do believe that when I have a relationship with a person, it's best in the spirit of love and honesty to express a change to her. "Thank you for your time, but this is not working out for me, and I'm actively looking for another sponsor who's a better fit," works just fine. No finger-pointing, no blame game, no "firing."
Just my two cents. I'll say a prayer that you find the person who's right for you.
Peace & Love,
Sugah