Op, you are lacking any understanding of just how frightening a cancer diagnosis can be.
When you are told you've cancer, your very first thought is that you are dying, that you are going to leave your children and family. That all your plans for the future will not now happen. It haunts your every waking moment. The fear is overwhelming. Fear of dying a slow and painful death that no-one can prevent.
Then you go through years of unpleasant treatment, and have legions of tests. Then if you are very lucky, you are told you are NED. This means no evidence of disease. It does not necessarily mean you are cured because no doctor can be sure of that. There is no absolute test. No-one ever tells you that you are cured.
After that, for months, every twinge, every ache and your first thought is that IT is back, that it has spread, that this time there will be no reprieve. The fear lingers. It's like waiting for the sword of Damocles to fall, all day, every day. If you get through the days ok, the fear surfaces at night. It's exhausting.
Psychologists reckon it takes up to ten years to get over the shock and fear resulting from a diagnosis. Some people never get over it.
I'm 3 years and 4 months in. I don't think about it much. I seldom talk about it and only with my closest sibling, but I am a positive, optimistic person, single, independent and self sufficient. I also have a full on career and a teen to keep me very busy. Don't imagine I've forgotten about it though. Others will take longer to regain their balance.
If you can't be kinder and more patient with your mum, then perhaps see her less. She almost certainly cannot help her behaviour. She is expressing her worries, telling you she is afraid. She needs your support, not your impatience and irritation.