I'm very happy that you're taking the time to sit still and question this relationship. So many girls drag these relationships on for years, while constantly trying to ignore their intuition to leave.
I have confidence that you will let go of him because your eyes are open to the problems. But you must realize that you cannot change him. Girls constantly try to do this, and at times the guy temporarily improves, but I've never, ever, seen a permanent change take place. This is because she is the one trying to motivate him. He always ends up back into his problems, because he is the one who needs to realize that his lifestyle is unhealthy.
Only he can change himself, and the longer you try to mother him, the longer it drags on his need to confront his behavior. If something goes bad, and he gets high to deal with it, then he's using that bad event as an excuse for his behavior. If you dump him and he goes back to meth, then again he's using unhealthy behavior to cope with his problems, and making up excuses. Nothing has changed.
If he goes back to drugs, it's not your fault. You're not putting the pot in his mouth. He just wants you to feel responsible so that you'll stay. He wants you to think that if you walk away, then you're basically leaving him with no other choice than to build a methamphetamine lab in his closet. Don't fall for this. It's a way that immature guys manipulate insecure girls. If he's that unstable, then he needs professional counseling, not a fifteen year old girlfriend.
If you leave him, it will teach him a very valuable lesson that his behavior has negative consequences that harm his life. It's the fastest way to help him grow up. But even if he comes back to you saying he's a changed man, do NOT go back. He needs a lot of time to get on his feet, and you need some time to find out who you are, and what you're dreams and goals are in life. In the meantime, pray for courage and strength, and do not be afraid.