I know the feeling bro....
I lost my mum to cancer in nov 2010,it was the hardest thing i have ever been through,You have to be strong for her and the rest of your family.
My mum's cancer had spread into her bones and then finally into lungs,the last few months of her life was extremely hard for her,but for us to watch that,and see her pain is not anything i can put into words. If you have FAITH then believe that when she leaves its for a better place,this is my faith and belief...that god will give us pain momentarily in this life,which is only a blip in comparison to the eternal peace of the hereafter.
Yes,when a loved one is in this kind of position...sometimes our own selfishness says that i wish they could remain with us longer..we forget the pain of the cancer patient..BE STRONG BROTHER and let nature take its course,have faith and a firm belief in your heart that god does not put burden on a soul more than which they can handle..
I pray for your MUM.