DECLARATION FOR MISTRESSES - Owning your Name

"I will not use his surname as mine when I'm not (legally) married to him."

You may not like the surname you were born with, adopted with, or that you carry from a previous legal marriage, but declaring yourself the owner of his surname when you haven't legally married is, in my strong opinion, out of the question.  (If you've established a career or a reputation in the name you have, it's totally out of the question.  If you simply hate your name, change it, but not to his!)

Declaring such possessiveness will probably get you nowhere, especially if it doesn't work out.  If you marry, then it's your decision to carry his name.  Let him gift it to you then. The younger you are now, the truer this is. Carrying on with a name you took in your youth, when there is no longer a relationship, may get in the way of you moving on into another relationship!