Is there a word that describes a person who once sets their mind on something it's unchangeable. But specifically, I'm looking to describe a person who once you do something that made them dislike you, there is nothing you can do to change their disposition towards you, no matter how many amazing things you do for them.

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An adjective that come to mind is "unforgiving."

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if you are really angry at them not forgetting and not forgiving towards you you can say something like.

"you are an evil cantankerous stubborn belligerent bellicose self centred selfish arrogant pigheaded bigot, and I rue the day I ever met you."  Don't be afraid to let rip and tell them in no uncertain terms how you fell about them.

self centered for American speakers I think.

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I agree with Jonathan that "unforgiving" is most precisely what you describe. "Embittered" is also good.

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I agree with all the adjectives. However, I found another interesting adjective "inexorable". An inexorable person is hard-headed and cannot be convinced to change their mind no matter what.

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They are adament they will not change their mind