Could you give up water? That is what it feels like for the alcoholic or addict. They may want to stop, they may see how their addiction hurts others, but to them the substance feels necessary for life.
So you cannot depend on an alcoholic or addict in active addiction. Whatever emotional bonds they have with you will some or most times take a back seat. I think conceptualizing it as selfishness does not help, because the intensity of anger that comes from such thinking keeps you attached. Better (IMHO) to say that they are in the grip of something neither of you can control--so you can walk away from the relationship calmly.