I am mostly Mistress, or my first name when we're in too public/ vanilla places.

But one time, when I was really sick, he called me his Princess, and that moved me so much that it became my nick and special private name that he only uses in special ocasions.

Besides that, he always uses You when speaking to me (which is more noticeable in Dutch then English, and can give funny looks from family when he forgets to be more familiar ) even though I am 10 years younger then he is