His letter to you expresses his love for you and his actions will show you his love for you is real--or not real. If he truly believes what he has written then he will get help for his addiction. Insist that he does so. If he wont, then you know who means more to him--you or his addiction. Then face the truth of whichever love he shows you is his true love (you or drugs). What do you want to do if he continues to choose using drugs over his love for you? Maybe he'll choose you if you make it clear you are the only option in order to stay in your relationship with him. It's up to you to decide what you can live with.