Why the hell did she go hysterical?

Maybe she knows something about your ex that she doesn't like or maybe she feels she's spent all this time and effort trying to help you get over your ex then you just go back to her at the drop of a hat. She probably just doesn't want you to get hurt again. I don't think she's a conceited anything she maybe thinks you're stupid because you're setting yourself up to get cheated on again. If you take back the cheating ex you're basicly giving her a free ticket to cheat again because she knows she can get away with it. I think you need to sit and have a calm conversation with her about what you feel and what she feels. at the end of the day she can give all the advice she wants but it's really up to you what you do with your life, i would just be a bit wary of the ex. be careful and good luck i hope it works out for you.