At least your mother went peacefully. My great-grandmother also died of cancer, I think lung cancer, though she never smoked. (Both my great uncles did and still do, sadly, and my grandfather smoked for years, though he's long since quit. I think my grandma also smoked, but like grandpa, she has also long since quit.)
Anyway, great-grandma's death was awful. I didn't see much of her suffering. One time when she was suffering, well, it was kind of dark humor. I had trouble not laughing as it wasn't funny. It's just that, in all my years of knowing her, I'd never see her, er, cut one, before.
It was REALLY loud too. I would have been laughing hysterically, had it not been because she was so weak that she had messed herself and that was the reason it had happened.
Anyway, her death was the rather painful sort where she couldn't eat well and was wasting away. She was my last great grandparent too.
I also lost my grandfather, on the other side of the family though, a few years due to cancer and he was going kind of nuts near the end. I wasn't there (He lived in Arizona and, in addition to being miles away, he wasn't always that close to the rest of us either. My great-grandmother was really close to us, but him, not so much.)
However, another thing is bothering me as of late. My first experience with death that I can recall is when my dog Pepper died of a stroke around age 9 1/2. That was quite a shock to me.
Well, we got a new dog named Noel that winter. I still have her, though, as I got her in 1999, she's now 15 and I can see she can no longer jump onto the couch, I see that she can't always get up the steps (her lime disease she got a few years back didn't help!) and that sometimes she seems to have to go out to pee (she can still hold it, don't worry) more than usual and I worry about her.
My worst fear isn't that she'll go like Pepper, but that someday we may have to put her down as she seems too healthy to have anything right now that would take her away peacefully or by natural means.
No, we're not in any danger of getting there, but still, I worry about it sometimes. In the glum mood I've been in lately with the state of the country, no job, and feeling like a worthless failure, losing my beloved dog is the last thing that I need so I hope she lives many more years and dies naturally in the end, if she has to die at all. (Due to her longevity, I've been calling her "Forever Dog" and have come to think I've prayed her into being immortal, you know like Binx from "Hocus Pocus".)