I feel selfish

it's hard to know how to write these posts, but i'll start with a brief look into my mum's history with cancer.

She was first diagnosed in 2015 with bowel cancer, had surgery and chemo treatment. It then came back in her lymphnodes and she had keyhole surgery for that. Then at the begining of 2021 she had radiotherapy for something on the lung, and then later that year it had grown to the liver - at this point the verdict was initially she'd have a year left even with chemo. However, she ended up getting the ok for liver surgery, had 80% of her liver taken out and regenerated (science is incredible) and some chemo to top it off. By now, we had been told that her cancer would be incurable and it would keep coming back. The next year the cancer had grown on her lung and again, she went in for another surgery. Since then she has had trouble with catching her breath and fatigue alongside arthritis. I've watched her deal with more and more health issues as a result of all the surgery and it feels like we have been living on borrowed time. 

Most recently, they found cancer in her lymphnodes and she has had her first chemo treatment today. We haven't really talked much about it, and when we have mum expressed that she worried about upsetting me if we did. In response I told her I'm going to be upset regardless if we talk or not and in fact, I'd feel better if we did speak and get it out in the open - she said she feels the same way. However, since that one conversation we have had I've kind of contradicted myself by avoiding speaking to her about my fears. I've avoided talking to anyone in fact - I've mentioned it to a few people in passing conversation when they've asked about her but not much else. That is until today, after I left the house to go to work, I walked onto the shop floor, my manager said hi to me and I burst into tears. It felt like the first time I really got to cry about what's going on, which felt nice to let out, but I feel guilty and selfish for it. I'm not the one who's going through cancer, I just get to watch and I feel helpless for it. and I hate that I feel this way because I want to be strong for my mum and I don't want her to see me cry or see that I'm scared. I want her to feel supported by me and believe that I can be her shoulder to cry on, not the other way around. 

I'm not entirely sure where I was going with this post, I think I just needed to vent and ask what can I do to comfort my Mum during her chemo treatment? How can I make myself stronger to better look after her?