Maybe she is best for him. With her, Tony went from undrafted, 3rd string, nobody, to the starting qb of the Dallas Cowboys. And from almost the very moment the Tony/Carrie stuff started flying, suddenly his honeymoon in this league seemed to be over. Literally she shows up at that game and Tony turns around and has his worst game to date. This nobody person is the one that was by his side keeping him on track to become what he's becoming at the moment. Carrie was like a leech who pounced on the headline goldmine we know as Tony Romo.
Screw Carrie (no pun intended), I want the driven Romo back! And if that's with this other chick, then so be it.
Another thing to consider, but maybe it was Tony that pushed her away. I seem to remember the weeks leading up to him replacing Drew as it being said he was being told to be ready. Parcells could have literally been pouring on the pressure as the days led up.