I've been in the exact same boat that you're in right now, Bene.
My grandmother ended up contracting a serious kidney infection, and on top of that she had learned that her heart was also failing her as well. I had a feeling that she would probably pass away rather quickly, and I was very, very scared. Every time I came back to her place, I was afraid I would come home to find her dead. I remember staying up all hours of the night to watch over her to make sure that she was okay.
I was there (alone) when she had passed, and it was probably one of the hardest things that I have gone through in my life.
If there's one thing that I tried my hardest to do, it was to make her last days on this planet as enjoyable as I possibly could. I learned quite a bit about her during those last couple of days, and those are some of the things about her that I hold near and dear to my heart (I'm the only one that knows the story about how my grandfather asked her to go steady on Christmas, and had her pick out her engagement ring on Valentine's day).
I hurts to think about that night, but at least she wasn't alone.
No one wants to be alone when they pass, and I think having someone there makes it easier to go. I'm pretty sure that your "mother" appreciates you being there more than what you could possibly know, and the fact that you're going out of your way to make everything pleasant is one of the best things you could do.
When it's her time, it won't be easy. There going to be quite a bit of pain, and quite a bit of tears and what not. As time goes on it'll get easier to cope, but you won't forget.
If you ever find yourself focusing on her passing, do yourself a favor and go back to all those good times and be thank that she lived, and that she was in your life.
Heck, if need be, write her a letter.
Feeling mad? Go ahead and scream. Best to let it all out then let it fester.
That's what gets me through all the bull.
You got tons of people here though if you ever need a shoulder to cry on.