@Capricorn68 at the risk of sounding a bit 'Pollyanna' (I'm assuming you are old enough to understand the reference) don't write yourself off as being not up to the new job quite yet.
Current stats suggest that 1 in 7 women in this country will be diagnosed with breast cancer. I think we can be confident that the majority of those will either still be in the workforce or have family who are.
There's a fair bit of coverage of brave struggles and tragic endings but you don't see much about the tedious reality of dealing with a life threatening event (yours or someone else's) while trying to plod through not just the next weeks or months but keep yourself on track for the rest of your working life.
It's not that long ago that going to work pregnant was frowned upon. Not appropriate. Those women who did it anyway set the standard. Attitudes and workplaces changed.
I like to think that those of us who are juggling cancer and work can, perhaps, change things for the better by hanging in there if we can. By taking the mystery out of it all. By proving that a couple of bad weeks isn't enough reason for anyone, including ourselves, to write us off.
If you previously had a good work ethic, nothing has changed. You are just under more pressure than usual and might need some flexibility for a while.
This can happen to anyone. And it does. Best of luck. Be kind to yourself and accept kindness if it's offered. Mxx