Female led relationships discussion

She feels she is too young to be called M’am, yet too old for Miss. We rarely use our actual names with each other; we’ve had adorable nicknames since we started dating. Recently, she prefers calling me Boy when we are alone. I need a public title that signals her authority without giving away the nature of our private relationship. I am proud of who we are and don’t mind others knowing; however, she is more reserved—and certainly my Mistress! Thank you in advance!