Karma,
Tell a little more about your ABF. How is he likely to react to being broken up with?
Chances are, no matter what his personality type, he will react negatively. If he has any dependent tendencies, he may do the "bargaining" thing, where he promises to get help, and "if you stay by my side I can get better", and all of the manipulative aspects. You need to be prepared for this, to take what knowledge you have about his reactions and prepare yourself for whatever the likely reaction will be. Practice it while you're still alone, out loud. Prepare yourself, with good sleep and food and deep breaths. It's like preparing for an athletic event...it's hard, and you need to be ready.
It may or may not be possible to be friendly at first (hurt feelings and all) but if you do this right there may be a possibility later of neutral feelings and even friendship (if you even WANT that...do you?).
Personally, if you're 100% sure you're Done, then I'd simply tell him what you've told us: That you're not comfortable at all any more with the choices he makes regarding drugs and alcohol, you do not want to be involved in the problems he has been creating for himself, you're taking your life in a different direction, and you want him to stop calling you. Try not to get angry, or to fall into the trap of arguing. This is a decision you're communicating to him, not an argument to decide who's right. He has the right to do whatever he wants with his life; you're just making other choices.
Block his phone number if he does not respect this boundary. Don't take an endless series phone calls trying to get you to see his side.
If you're **not** 100% sure you're Done, and you're willing to talk in say, 6 months, if at that point he can prove to you in writing that he's been sober (drug tests, rehab, inpatient) and self-sufficient financially, then you can do that too. But it sounds like his problems are deep-seated and are likely to be recurrent for his whole life...I personally would not choose this route, but you have to follow your heart.
Remember that if you give him an opening like "stop calling me for a WHILE" or "I need some time to THINK" then he may hold out false hope that he may be able to rope you in with promises. It may hurt to be so final, but from personal experience I can tell you that it's much more cruel to string someone along like that, even if you're not sure. Plus it creates the possibility for tremendous anger down the line on his part.
Anyway, from all of this perhaps there is something you will feel right about doing. I'm so sorry you're going through this. You sound like a good-hearted, level-headed woman with a lot of love to offer someone who's capable of giving it back.
Hugs to you, to do whatever you feel is going to bring your life the greatest joy.
GL