So sad and sorry to hear this. I can understand the pain and anger you are processing. I have found it best to just accept, then to beat yourself to death trying to understand why.
I have heard that same story so many times, I pretty much knew what the next sentence would be. Even with having watched several folks go down the path, I still find it hard to imagine how someone can have it so well, loose it all to drugs, and not be scared out the wazoo. The lifelong grip that drugs have, is unimaginable for the rest of us.
I have seen it a million times, Mothers, & Grand Mothers have a hard time saying no, Even when they know it is wrong. You have already lost a son, Do not write off your mother before she is gone. Her time is limited, Savor what you can. I know this is a hard thing to forgive, but please give it some thought. Maybe have a talk about your feelings (I can't believe I just wrote that, I am the type to keep everything bottled up. It cost me a relationship with my Father).
We will pray for you and your family.