Hi Ellie,
I am so sorry to read of your mum's diagnsois. I must admit I was drawn to the thread by her age - as I was 47 at diagnosis, so wasn't really expecting to read that she had lived with this for so long without any support or medical help. I don't know if this will help, but I have a friend whose mother did the same, albeit she was a much older lady than your Mum (in her late70s/ early 80s). She, too had drugs and radiotherapy and went on to live a happy, healthy life for about five years unitl 'old age', not cancer, took her away.
Staying positive is definitely a good thing, and the doctors will do everything they can for you Mum to get her well again. She may well be feeling guilty that she didn't do anything sooner - but that actually won't help, and can't change anything now. The importnat thing is to concentrate on the 'now' and take whatever help is offered. Make sure that you get good support too - try BBC helpline or speak to MacMillan.
Best wishes for all of you, and a gentle hug,