I want to agree with Doc Durian and say don't look back and go find somebody else. I want to agree with other posts of people saying "Never look back at exes, that's just you being weak." However being a complete 180 reformed individual and human being from my recent experience, I can attest that people do change. She could become the person that you want in due time.
How much time though? Nobody knows and that uncertainty is what is keeping you kind of hooked and wanting to talk to her. Along with the pain you are feeling, you want that to go away. And no matter if she changes or not, she might not want you in the same respect. That's why it's very discouraging to look back
Being with somebody else is a crutch to keep you from changing, because you have somebody that will be with your currently no matter how unhappy you are. Until somebody else comes along. As long as she is with somebody else, her change is inhibited. It's much more appealing to try and find somebody else and with your experience you can make it happen right the first time. Who knows what will happen in the future.
Your problem seems to be more of finding the wrong girl than being the wrong guy, and that should be something to keep your spirits up. Too many depressing stories about being an asshole that ****s up and just pining for her back when the damage is done. You will find a good person and be happy with them. I'd put money on that.