Ok i have asked a lot of frivolous questions but this is a very serious one?

a girl did something that made me very very angry. she worked with my boyfriend and tried to get with him, using every trick in the book. he resisted for 3 months but 1 night got hopelessly drunk and let her kiss him. i know him so am not asking for your judgement on him, i have sorted that myself. the problem is that months on, i have a red hot burning rage evrey time i think of her. i think i am a good person, but i feel real hate whenever i think of her. it's eating me up. i lie awake at night having imaginary confrontations with her, and don't like myself for doing it! my relationship with my boyfrind is great and has been ever since this 'blip' months ago but no-one realises the rage and loathing i have inside me. i'm completely obsessed. i hate hate hate her. i want to get rid of it. how?