I know it's a platitude, but a true one, that it's still early days. You will go through a great many emotions over the grieving period, and anger will be one of them. You truly have a lot to get angry about, but eventually that will subside too.
Your mother was one of the unfortunate A's that can't kick the habit before it becomes fatal. It hurts everyone around them, and themselves most of all.
You've done what you can to have a healthy reaction to her behaviour and learned a lot about how to cope with life and responsibility on the way. This will make you stronger in the end.
I've had people I know commit suicide, and I kept beating myself up with 'what if' I'd done this or that. Nothing I could have done would have saved them; I just had to admit that we are powerless to control others.
I hope you eventually marry and have a lovely MIL. I'm very close to my DIL, and anyone would be proud to have you as their daughter. I'm sure your mother was too.