I’m very sorry to have to be the one to tell you, but you do need to get out of this relationship. It’s not going to get better. Your boyfriend’s relationship with drugs is more important to him than his relationship with you. Please notice that he is more willing to risk your relationship than to break up with his habits.
I don’t know why this is so. It doesn’t matter. You and I don’t have to understand it. He does. Then he has to find the motivation to truly change. I don’t think it’s likely from what you said in your letter.
Length of time in a relationship is not a good reason to stay with it when there is no trust. You are only 19. You have lots and lots of time to date other men and to find someone whose primary interest and concern is with you and your relationship. You deserve to find a man who you can absolutely trust. Don’t settle for anything less.
I wish you well.