What Neddy said. I've been in your situation before, and I've been in hers too, so I feel qualified to say this. Don't ever try to rationalise with her; she knows that what she's feeling is illogical, and it's only going to make her feel worse that she can't fix it. Just be there, and offer a hand or a shoulder when she needs it.
If it really gets serious, though, it would be extremely wise for her to see a proffessional. You might want to help her understand that a lot of people go through what she is feeling, and that there is no shame in seeking help. You may also suggest that the root of her insecurity is chemical, and that a proffessional may be able to help her with a drug to get her brain back the way it's supposed to be.
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Strewth
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