Were you truly looking for support, or, was it just for sympathy? The way you described the situation, sounded like you feel innocent in all of this ... the victim of an ex-gf that wouldn't accept your sudden changes and went crazy on you. It really didn't sound like you felt responsible in any way. I think that is what the few people who posted before me were trying to get at.... Do you realize and take responsibility for the part that you played in your situation?
The reality is, if the girl became a monster ... it was one that you helped create. Im not being judgemental when I say this, just trying to help. I think the only way you would understand how she reacted is to put yourself in her shoes. If you truly love her, she needs to know you cared about her feelings, and see you are sensitive to her needs and not only your own. Giving her no choice but to go along with your wishes would cause resentment. Naturally, if it was building up in her she would start seeming a bit "psycho". Further adding to her pain was that she was dumped and alienated. Did it occur that maybe she reacted out of sheer pain? She must have loved you alot, to even agree to let you have the other girlfriend even after the cheating, and to try to go along with your wishes. She was willing to try, even if it wasnt what she really wanted, if it meant not losing your love. That says alot. Yes, you lost... alot.
"The heart wants what the heart wants." -A Walk In The Clouds