Hello there 

So sorry to read this message. 
I was just considering your job situation. I'm assuming you're in the UK? 
 

If you have resigned, there's absolutely no harm in telling your employer that you resigned in haste, when you weren't in the right frame of mind (due to cancer and treatment) and that you would now like to rescind your resignation. If they're good in HR, a mention of a 'heat of the moment' resignation, when an employee was recovering from cancer treatment will very likely *** their ears up. 

You can also tell them that your cancer and treatment have knocked you for 6 and so you wish for them to consider reasonable adjustments around your shifts (I.E only assign you shifts that aren't earlies or lates). They're obliged to consider adjustments (you're heavily protected under the Equality Act 2010). 

Even if you've resigned with immediate effect, you could still try this approach (...I'm thinking...in light of everything else that's gone on, it might be better for you to be in that job still so you'll at least have an income...and if they can make adjustments around your shifts, it may well be easier to handle). xx