I am so so sorry to hear this. 
I do not mean to come across as morbid or negative in anyway so please forgive me. I lost my mum a few days ago. She was 53 and she also had stage 4 cancer. She battled for 2 years. I am numb, empty, lost, confused and don't believe it was real. 
my advice would be to not spend your time worrying. Believe me I know its hard, but what is going to happen, is going to happen and the very last thing you need is for when the inevitable does happen is to be filled with regret that all you did was waste the last few years worrying and being down. Enjoy every single second you have with your mum. Watch TV together, tell jokes together, bring up classic memories together, eat together and just love each other. Nothing else should matter! We all cope in different ways and have different situations. But when it comes down to it we all love our mums. I maybe wrong, but from my experience it still now doesnt seem real. Even after years of expecting. I have my doubts and regrets that I could have done more and I will live with that for the rest of my life. And that is far worse than the worry I had of when she is gone.. might just be me as I seen her nearly every night or certainly every other night. In regards to feeling scared for your mum just take her mind off it. Be with her, talk to her and support her and treat her how you always have done. I'm sure your mum loves you and the last thing mums want is their own children feeling scared or afraid. So don't feel scared or panic or anything like that. Live in the present! Not the future, and by doing this day by day and taking each step as it comes you can just simply enjoy the company of your mum. 
Forgive me if this hasn't helped or come across wrong. I have good intentions. I just don't want anyone to feel the numbness that I do and overthink your thoughts when the day comes. And who knows that may be a very long way off.  

Hi [@Jake0968]‍ , I'm so sorry for the loss of your mum, and my thoughts are with you right now with what you are going through, it must be the worst experience ever, unfortunately shortly I will be experiencing it too. I felt like I needed to comment as what you have said is exactly right. Your words have helped me, I'm losing my mum to this horrible disease and she only has a few weeks left. She is 57, battled stage 4 for two years also. I want to Thankyou for your advice and I hope the lady you have responded to takes it too. Xx

Hi [@Jake0968]‍ , I'm so sorry for the loss of your mum, and my thoughts are with you right now with what you are going through, it must be the worst experience ever, unfortunately shortly I will be experiencing it too. I felt like I needed to comment as what you have said is exactly right. Your words have helped me, I'm losing my mum to this horrible disease and she only has a few weeks left. She is 57, battled stage 4 for two years also. I want to Thankyou for your advice and I hope the lady you have responded to takes it too. Xx