I don't think she is doing it deliberately, I think she has never practiced saving money, does not know how, sees what she wants and buys that, etc.

I would call her bluff on her promises and go straight to "write me a check for $50 every payday to put in the saving-to-move-out account." Make the goal very clear-cut and unavoidable (and interesting/exciting -- start apartment hunting together?). If she will not work with you when you specifically and point blank ask for help, then I would decide she was not the partner for me.