Jungkook is your boyfriend of five years, he loves you a lot and you love him too, but you have a twin sister who is jealous of your relationship, so jealous that she locked you up and acted as you... and Jungkook fell for that.
When you got out and tried to go to Jungkook he thought that you were the twin and just pushed you away, and went to your twin and kissed her.
You felt hurt and betrayed and decided to stop trying to get him back.
One day your twin told Jungkook that you were planning to kill her and that he had to kill you before you could kill her, Jungkook believed her because he still thought she was you, so he went to your room at night with a knife, ready to kill you.
You didn't fight back instead put both of his hands on the knife and pressed it close to your heart, letting him kill you if he wanted to.