I honestly can not believe this is still going on. Teenie, he is making a fool out of you and every time you 'sit him down' or 'have a long talk' you are allowing this to happen. Every time you make an excuse for him you give him permission to hurt you all over again. He does not love you, he doesn't even like you. If he did he wouldn't do these things.
Right now you are kidding yourself, whether you choose to believe it or not. The ladies on here have been so supprotive and offered you some of the best advice there is and you are choosing to ignore it.
He does not need to be told he is choosing drugs and this girl over everything else - he knows! He is making a conscious decision to do so! You don't need to tell him he will never see you again - just don't see him! It will not make the blindest difference. The best you will get is him crying (which will not be genuine) and telling you he will change (which he won't). He is taking the piss out of you good and proper sweetheart and you are allowing that to happen therefore you have to prepare yourself for the pain of one day realising that this boy has been playing you and laughing behind your back about it.
The day will come when this has to stop, for your sake i hope you get a grip sooner rather than later. I hate to sound patronising but when you get a bit older and find a real man who you don't need to run around after and make 'plans' to get back, a man who treats you with respect and looks after you, you will realise just how stupid and blind you are being right now.
WALK AWAY