This is a tragic story, I have tears in my eyes reading this and felt the need to respond. I’m so glad you felt able to write all this down and get some pain out. I can’t imagine what you are going through but needed to assure you that you are absolutely not to blame for anything.
These forums are good for opening up if you feel unable to do that to people close in your life.
From reading your story it seems that despite everything you are going though you have graduated from uni and are managing to keep your life on track which is admirable and shows your strength in character. All you can do is be there for your mum as she needs to want the help and sounds like she is not ready for this. Addiction is not easy as I have watched family members go through this myself- I hope your mum finds the strength to beat this one day. Do you have a good circle of friends around you and you spoke about you nan, do you have a good relationship with her to support each other? Life is cruel and you sometimes not everyone can be saved but you can choose to save yourself. I’m here if you ever need to chat more x