I know I am older than you are, but I am a scared daughter too. I was told that I would lose both of my parents on the same awful day in August. I too wish my mother had never picked up a cigarette. I'm not sure which parent I will lose first, but each day is difficult.
I don't know what type of cancer your mother has, but I found help as to what to expect from www.brainhospice.com. The stages of death are spelled out so you know what to expect. There's no BS. They tell it like many people have told me will happen.
As far as your mother not knowing. My mother has not been informed either. My dad was told stage IV and they used the words terminal, but nothing since August. I'm not sure how much to tell either of my parents. I do know that my dad is full steam ahead with radiation, therapy and rehab. I however, see many of the signs of death approaching. If I say it aloud, from what I've read, he'll deteriorate and give up sooner. So I've kept my mouth shut so I can selfishly take in every conversation, everyone one of his silly jokes for as long as I can.
Try to enjoy your mom. Try to stay calm. Try to smile.
I'll be praying for you.
Cheryl