She is not your best friend if she really intends to damage your reputation, otherwise it might be your misperception or taking things seriously. First of all, define what is nasty. Does nasty things mean sharing your personal matters or secrets with others which you don't like or putting accusations on you or spreading baseless, disgraceful information about you? If she is spreading baseless, disgraceful information about you, just forget it. If she is seriously disclosing your personal secrets to others with malign intentions, talk to her and make it very clear to her that she should not do this as a best friend.Good luck!

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