@MicheleR. The Cancer Card is a tricky one to play. A bit like The Joker, there are a limited number in the pack.
You can, of course, be playing multiple games with multiple players, or restart a new game with a previous payer. You've got to keep count of when you've played it and with whom. Sorry if that sounds strange, but it's the best way I can describe it. Taking advantage is wrong, but it is a genuine game changer.
Essentially, the Cancer Card gives you licence to say 'No'. Just no. No explaining, no excuses, no deals, no negotiation, no trade offs.
'No, I am not going to be able to do that.'
Sometimes we are are own worse enemies when we try to keep everything as normal as possible for other people who are resisting changing their behaviour to accommodate, in even the smallest way, our really nasty circumstances. It's always at our own expense when we try to appease these folk.
A reasonable 'No' will be accepted by a reasonable person. Mxx