Most Helpful Girl
If she's telling the truth? It probably has a lot to do with feelings of guilt or inadequacy. It sucks to be in a relationship where you're constantly asking yourself " They're so great! What are they doing with me?" or "why can't I be like that?". Nobody wants to feel bad about themselves ALL the time (or even some of the time), I think it would just be tiring constantly feeling like she wasn't a good enough person. E. g. If you guys were out, she probably thought other people were asking themselves "What is HE doing with HER?" .. In her mind, if she loved you, she's probably thinking, NOW you can go find another girl, who's better than her and deserves you. Likely she thought she was doing what's best for you.
I think if someone is in a really good relationship, then every now and again, they ask "what did I do to deserve this person?" It takes a certain level of trust, maturity and self-esteem to move passed that guilt feeling, and accept that they have it good.
Your partner has to trust in your judgement, that you know how to pick the right person for yourself. They have to be mature enough to understand that a good relationship can be hard to come by. And they have to have enough self-esteem to feel comfortable with someone who they feel is 'out of their league'.
If your ex is your age (16?) or younger, then it's not really uncommon for her to not have those traits. Self-confidence can be pretty crappy at that age, so don't think of this situation as likely to happen with every girl out there.