There is no 'polite' way to say, "Mind your own business". More polite ways to say the same thing are numerous and varied, depending on the situation. Some possibilities:
1. "This is a private matter between ____ and myself. Would you excuse us please?"
2. "I appreciate your interest, but I prefer to handle it in my own way."
3. "I want to try it my way but, if I need help, I know just where to turn."
4. "Go peddle your papers!" could be said to be just as firm, but slightly less confrontational- more jokingly.
Student or Learner