Please take a good read around here, trying to be "the one" to save an addict is a dangerous and troubling place to be because we can't. If love could save an addict, not one of us would be here, it's up to them to change and if she doesn't think she has a problem then she isn't likely to change any time soon.
It's your choice to make when you're ready, to continue a life that will break your heart or to stop while you can. Reading Codependent No More is a great suggestion , many of us learned so much from that book, and if there are any Al-anon, Nar-anon or CoDA meetings in your area, maybe give them a try and see if they don't help you as much as they have helped so many of us.
If I may suggest one thing, please do not take her to meet your mother if she has cancer and is taking chemo, she probably has other medication to help her through the pain and I promise you that your girlfriend will steal these medications and swear she didn't and leave your mother in pain. Most active addicts would do just that.
Like I said, please take a read around and learn from those who have been where you are.