I suppose “Flavor of the Week” is out?
When we were dating, my wife was my “sweetheart”. Even now, after 7 years, I call her sweetheart more often than use her name. (That reminds me, I should find out her name again… 
) I never could do SO or girlfriend, but I did use fiancee when that was appropriate.
Sweetie is reserved for female friends. I don’t use the diminutive for my sweetheart. Wonder why that is?
Eliphalet