I am not a professional, but I think I can point you in the right direction. Your life will get much better as soon as you can put this man out of your life and keep him there. You say you have come to terms with his leaving, but that it is ruining your life.
Professional counseling will help you put his leaving in perspective, help you get your self esteem back and get you on the road to healing.
This man is a drug addict. Even though he cut down on using in front of you, he probably still used more when you weren't around. You have no control over what he does. He is responsible for that, and he is the only one that can take care of the problem.
You can love someone dearly, but you have to learn to accept the things that you cannot change, and go on with your life. A professional counselor will help you with that.
Try to stay busy and get involved in other activities that do not involve him or his friends. You will find new friends and new interests and very possibly find someone without the drug problems.
I know it hurts and is hard to give up on someone after being together for that long but you will learn that there is nothing you can do for him. It's time to do for yourself now and let him go.