Play it cool. Relationships from college into real life don't exactly have a huge success rate.
I wouldn't put more into the "guy friend" thing than absolutely necessary. Date, live your life, etc. She may be clinging to the comfort zone of her old fling, but exploring her relationship with you. Or she may just be one of those girls that feeds off the extra attention of a guy in her hip pocket.
Maybe she displays a little jealousy when she sees you moving on. That's your opening to let her know that you'd rather it was her, but you have no choice but to get on with your life.
Courage is not the absence of fear but rather the judgment that something is more important than fear.
The brave may not live forever but the cautious do not live at all.