An update. I never said anything about her checking out other guys (its what good looking people do), but as many of you pointed out, it's a symptom of disrespect, and the disrespect will manifest itself in other ways. I had to pay for everything so yesterday I called her out on it and nexted her. She was shocked. She has since completely changed tack and texted hinting at a reunion, which I dismissed.
I only happened upon this forum and The rational Male as I'd been having trouble with this girl. The Rational Male blew my mind. I know now that from the start, she should have been in my frame. I was in hers at the start, though recently she was in mine. I allowed disrespect from the beginning, and it's very hard to roll that back.
Ironically, the only way to fully gain her full respect was to end things. Onwards and upwards.