How do I keep my lovely mum positive when I’m crumbling inside

Mum has just finished 6 sessions of Caelyx for her cancer that she’s been living with since 2022. This is her 3rd lot of chemo and she’s actually been fairly well throughout, always got up, showered, dressed etc always functioned pretty normally. We’ve been told her bloods show her CA125 levels increasing on her blood which isn’t usually a good sign, I know this information, as does my dad and my siblings (who aren’t that supportive, they are much more matter of fact than I am and I struggle with my emotions, whereas they are a lot more ‘together’). My mum doesn’t know this (which feels like betrayal but I know it’s best she doesn’t know). I don’t know how to cope with the situation going forward, my mum is my best friend and I really don’t know how to remain positive or keep it together when inside I’m crumbling. Sometimes I have to stop myself crying several times a days, even writing this post I could cry my eyes out. I don’t think they’re keen to try her on another chemo as she could be very unwell on it, it feels like the beginning of the end and I honestly don’t know what to do. She will have a scan next week and then results soon after and I have an awful feeling…