How can I get my girlfriend to stop hanging out with and talking to her ex boyfriend?

She says I mean the world to her, and I believe her, but she won’t stop hanging out with him even though she knows how much it bothers me. She says its because he is nice to her and that he is the only one willing to go out of his way to hang out with her since she doesnt have a car. She lives 3 hours away from me so I don’t get to see her as much as I would like.