Anchorhead, your mother may have had a cavalier attitude to smoking, my parents did too (though I am a lot older than you by the sound of things) but when I was a kid I was hardly ever indoors when they were smoking.
They were either at work (and boy did they work to keep a roof over our heads and food on the table) or I was outside playing or in the library reading or in school.
It's horrible what you have been through, it's no joke to have a parent die young from anything including cancer (something we have in common) but without her you wouldn't have life at all.
She must left you some redeeming memories?
There are plenty of things all of us do which we maybe shouldn't and that may have an adverse effect on the people around us but that's not the whole person.
When my father was diagnosed I felt awful for him (he was only 49) and for the rest of the family too (most of which did or had smoked at some point in their lives and often with children in the house) because he was my father.
Even to this day I see something or hear something that he would have liked and I want share it with him but he's dead and if he hadn't of died of cancer he could have died of something else just as easily and I'd miss him just the same.
We don't even know if his smoking gave him the cancer in the first place, we have a history of it in the family and it's effected people who didn't smoke.
Whatever you do you will die and you never know when.
It's so important to embrace life and enjoy it and the company of those around you while you can and cherish the good memories.
Use the bad memories as a warning but don't dwell on them and let them poison your finite life.