I'm sorry you're having to go through all of this. I am in a pretty similar situation as you, only a few years older. Lived with my mum alone for 10 years. Only recently have I stepped back and I am now splitting my time between my mum and dad's place.
I TOTALLY was in your shoes of fearing what would happen if I left her alone. But. Honestly, she's going to do what she's doing whether you're there or not. This kind of thinking can become really pervasive, what happens when it's time for you to live your own life? Will you choose to stay back because you need to ensure your mum's safety, essentially taking responsibility for her life?
Basically. We can't save our mums from themselves. We can't. It hurts, and it's scary. But putting your own life on hold for the sake of your mum's won't help you or her.
My mum also lives in the past, vents her anger about my dad to me, isolates (she literally has one friend who she sometimes talks to on the phone, otherwise she doesn't interact with other people than me), has major depression and anxiety and now cirrhosis. The thing that might help you is to know that she probably doesn't want to be doing what she is doing, but she is literally powerless against it. There's a lot of shame and self-hatred felt by addicts, but no matter how much prodding and pushing anyone does, they will not back down. It has to come from within, and they need to feel the consequences of their actions before it can come from within.
If you're there to clean up her messes and ensure her safety, how will she ever learn she has a problem? Or feel motivated to change?
Do what you need to for yourself. Keep yourself safe. Do at least one thing each week JUST for you, ONLY for you that makes you feel good. You can't be of any help to anyone else if you aren't of help to yourself first.
I know your pain, you're not alone in this. It's so, so tough, but I believe in you.