Try to find a way to remain on the high road while confronting the situation as calmly as possible. Right now, you are both building camps to save face or be right rather than being direct with one another. Being respectfully direct will get it out on the table and you can hopefully both move on feeling relief. An approach might be –

“Hey X, I notice you continue to bring up Y, so I think it is important we discuss this. What you are saying is untrue. What actually happened was Z (and only to the point you are comfortable sharing that.) When you share something about me that isn’t true that hurts me and hurts our relationship, which I value. Do you understand how I feel? Is there something you’d like to share that I need to understand? Can we agree to drop this and move on?”

Allow X to save face while you accomplish your goal (end of destructive gossip) and model the type of behavior you’d want someone to confront you with. Being an introvert, practice your approach and role play what the exchange might look like so you build confidence. Good luck!