I use it to describe my Thai partner, actually fiance. If we were married she'd be my "mia", but as we're not, she's my "teeruk" (tilac etc).
It sounds as though she's your "mia" already. Surely [F]thii[H]rak has more to do with whether it's love or just lust on your side?
She is... sort of... we just haven't formally been wedded yet... soon this year
We call each other "teeruk" all the time... never by name.
I do however say to other Thai girls that approach me "phom me mia laeow", just so they get the message that I'm not on the market.