if only u knew the power you have in ur hands right now.
what i would do is treat her as a jump off. dont go to a movie or dinner before u fucck. each time u meet, make sure its to fucck. dont do anything that does not involve sex. no giving her rides anywhere. no meeting up for lunch during work hours, unless she blows u first. and whenever u do mess around, always kick her out. meaning, dont be cuddling and all that bullsh1t afterwards..
and always keep urself busy or give the illusion that you're busy. with what? that should be and must be left to her imagination. NEVER EVER tell her what u got to do when she wants to stay longer with u. just tell her u got business to handle. dont punk out on this. women have a way of pressing u for information. dont give in.
i typed all this because i can tell u obviously still have feelings for this chick. if u want to make sure she remains urs for good, this is what u must do and must continue doing otherwise, u'll have to deal wiht her leaving u again.
and most importantly, dont cancel dates on ur other chicks for her.