This!

I have a so called artist name. Just the official designation of a legal alias or - in my case - pen name. It's an actual, usable name, but you can view it as a title, if you like.

I do use my alias pretty much always and everywhere. It just turned out like that. But my mom doesn't use it and doesn't have to. To her it's just what it essentially is - a stupid nickname.

Make your former name your secret, intimate, personal name only your gf can use. Nobody else is even supposed to know, so in public she'll have to use your new name. Sell it to her, so she can realize that she's the one who has something special with you.