Most Helpful Girl
It's okay to stand your ground if needed and not cater to bullshit, demands, etc. There is always a place for some level of sensitivity though, and maybe that's why she doesn't feel as if she's good enough. You see what you're doing as having a backbone, but she may kind of see it as a cold shoulder or lack of emotion, commitment... things that would make her feel insecure and not good enough.
Things changed because girls (and I'm assuming guys) expect some sort of shift once a relationship begins. Once a relationship is established, so are different sets of expectations. If you want her back, try talking to her about it. Ask her WHY she feels as if she isn't good enough. Maybe it's something you unintentionally did, and maybe its her own issue. If it was because of you, just explain your position - let her know you never intended on making her feel that way. If its more of a personal problem, listen to her, comfort her, and let her know that YOU think she is good enough. Maybe that's all she needs, reassurance.
Most Helpful Guy
She can't be the only one to make you feel good; you have to make her feel good, too. It's a partnership. You've got to exercise a little grace sometimes, because some people cannot handle being judged for what is the truth if you show very little signs of actually caring about them. It's happening the same way every single time because your attitude is the same way every single time. Change that, and the girl who is best for you will stay with you through thick and thin.