Hi Tlo, welcome to SR. You've realised you can't help your AB, and taken the only course open to you, which is to move on with your own life rather than go down with him. He's been warned about his future by the doctors, and all the love you gave him couldn't pull him back.
Going non-contact is usually the best way to make a clean break, especially if you're susceptible to relenting. Your guilt may be natural but it's not logical. The truth is you're only peripheral to him because his focus is his next hit. You're not abandoning him - you've tried for 3 1/2 years after all, but at some point you have to make a healthy decision.