Why do some girls think they don't deserve better?

When a girl meets a guy who finally is going to treat her right, why is she quick to say we shouldn't be together? Is it her low self esteem that's truly talking? after getting hurt so much, is that all these types of girls know? Will they ever feel safe going out with a guy who will respect them?
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How can I get this girl to trust me...she always tells me that she feels safe with me, but when she gets stressed out...she starts doubting...she keeps saying that she doesn't think we should be together...is that low self esteem?
Why do some girls think they don't deserve better?
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