When drugs are discovered, it's natural to think that the elimination of the drugs will stop the behavior and solve the problem- but that's a big NO when it comes to personality disorders.
For most people who have a personality disorder, their coping skills are so nonexistent, that the person sometimes has to turn to drugs or alcohol to give a sense of relief from other people. For the most part, BPD comes first and addictive response comes later.  :)rugs or alcohol are what the personality disordered thinks is going to save him at first- but it is a fool's ruse. Many unsuspecting victims of personality disordered people think that drugs and alcohol are the cause of the personality disorder. Drugs and Alcohol it seems, are the tip of the iceberg.
If you've tacked on the BPD to make sense of the this drug or alcohol behavior- that's something that many people do when they are searching for answers to why their loved ones seem to be using and abusing substances. The reality is that the using is in response to something else- generally society. It may not be a personality disorder- it may just be a phase and a coping mechanism to get through the phase for lack of a better alternative. Either way- it has nothing to do with your protective measures to stop their behavior. Your job is to ask yourself why you dont give as much thought to your own future- as you do the addicts. In a way, this is a fork in the road that allows you to see that you're both struggling with the same problem: Addiction.
Your addiction is to another human being. His addiction is to a substance as well as to you. Both of you need to find your center core and realize that everything you seek is within you and not found outside of yourselves (what's outside is afraid and taking leave on a whim.) When you realize that both of you have some things to figure out- that you're not being left behind, you'll feel unafraid and you'll be able to let go of the other.
Take the path that leads to the unknown- not the one that circles back to him. He's got to do work on his own and you've got to stand solo. To answer your question; it doesn't matter if it's drugs or a personality disorder talking, what's left to answer is the reunion fantasy you have with each other. This is your work. It's also his. You cannot fantasize that he will emerge out of love for you and overcome his addiction. He has to do it for himself- not because of you. You also have to do it for yourself- and not because of him.