He may die if you leave him. He may die if you don't. It's hard for a newbie to comprehend, but it isn't about you. You don't have that power.
You didn't CAUSE it.
You can't CURE it.
You can't CONTROL it.
It's his choice to make. And frankly, if you gave the guy a choice right now, you or heroin, he'll pick heroin. Oh, he'll tell you it's you- but she's the girlfriend now. And he will tell you anything you want to hear to protect the addiction.
Somewhere out there is a nice guy who is not an addict who will be thrilled to be with you. Go get him.