Relationship Talk

So I'm dating this guy. And we're pretty serious, but he had a "girl" friend he has had claimed to be friends with for years.. and he won't drop her. She's tried to come between us many times in just 2 weeks, and every time me and him talk about it he gets defensive. What do I do? And this girl isn't very pretty or anything special, espically since she has been around. I need help😒😔