How do i cope?

Hi, my mum had first got cancer when i was 15 (breast) anyway she fought that and was near remission to then be diagnosed again!! Cut a long story short she has it again for the 3rd time but incurable shes been havin treatment 5 years its under her arm (lymph node) sternum and now recently spread to her lungs, im thankful that shes made it 5 years having treatment and still being here but im finding it hard to see it progress and spread and now seeing the effects (mums breathing is affected now) i just cant imagine my life without her and im finding it hard knowin one day she wont be allowed treatment and i dont want to see her in a way where she deteriorates, i cant stand to lose my mum she means so much to me and im so close to her knowing one day i wont get to ring my mum or go see her kills me, im 29 f and have two children single mum, i know my mum is upset and scared but she tries not to show it, i just dont know how to cope with it i think about it alot and cry thinking what will come