What do I do about my hurtful ex boyfriend?

My ex broke up with me 3 months ago. It ended pretty bitterly. We still have a lot of mutual friends. Apparently he has been saying a lot of negative false things about me to which his friends tell me and tell me they already know hes lying. His best friend has told me repeatedly im really good for my ex and hes an idiot and im a good person who did nothing wrong. My ex tried to make me feel like crap when we broke up. Why is my ex still talking negatively about me when he said he wanted nothing to do with me??