Spank me if you think this isn't true, but, my opinion is that if a guy breaks it off with a girl, then she is a wreck - huge self confidence killer. Therefore she constantly thinks "what's wrong with me" and pines for the ex degrading herself along the way and not allowing anyone else into her life. However, if she is the one doing the ditching then she is over the boyfriend immediately.
Unfortunately for you, she was the one ditched.
My advice is be there for her, enjoy the ride so to speak, but I certainly wouldn't put too much effort into a relationship until she solves her issues and the ties to the ex.
A lot of truth in this. When a girl does the dumping, by the time it gets to the "it's not you, it's me" she's thought about doing it A LOT and has pretty much made up her mind, so trying to work things through rarely works.
On the flipside, when a girl is dumped, as you say her self-esteem takes a big knock. Some girls (paradoxically given the above) try to fight to try and save the relationship to the point of desperation, whereas others will go out and try and move on too quickly; either via a few one-nighters or some unfortunate guy who's been the shoulder to cry on for her.