Hi SKA. Lots of advice on here, most of it against this relationship. Easy for us to say from a distance, but sometimes you cant see the wood for the trees. If this guy was a dealer, he has obviously mixed with some nasty people. He might want to change, but they might not let him. He is being very unfair putting the pressure on you to keep him off drugs. If he really wants you, he should be saying , nothing in the world would tempt me to go back to this evil way of life.
Had he not tried to blackmail you, i would have been more sympathetic.
At 19 there are LOTS of 'sweet' men out there for you - without the baggage !
Good luck hun !
x