Rip off the plaster. Message her tonight.
Hi CF, I've realised we never talked about how long you'd need the emergency lifts. Tbh I thought you'd have the long term arrangements sorted by now. So to be clear, I'm happy to give you lifts for the rest of this week and if you are really desperate part of next week too. You probably already know this but you can get travel help through . Don't worry about giving me petrol money for the lifts so far, a bottle of something do well enough! See you tomorrow.
A message is better than a conversation. It gives her a chance to be shocked, moan and rant in the privacy of her own home, get it all off her chest then realise she's a CF and be all nice and polite with you in the morning.
If she responds to suggest that you are her long term solution then post here for reply suggestions. If she tries to guilt you in person use silence and kicking the can down the road.
"That's not what I expected. I'll have to think about it."
"Ah, oh dear, that's awkward, um, I was only offering help for your immediate crisis while you sorted proper arrangements. I'll have a think about whether I can keep it up or not and get back to you later."
"That could be a problem. I'll have to sleep on it."
"No, really, I can't decide or discuss now, I have to think about it and discuss with DH."
"Oh, we might have crossed wires here. I'll have a think about what I can do and let you know later tonight."
"I understand what you are saying. I have to think about my options. I can't do that now. I'll get back to you later."
"If you must have an answer now then the answer is no, I can't agree to any more right now."
Do not JADE your decision with her: Justify. Argue. Defend. Explain.
Make sympathetic noises about her problem, while making it clear it is her problem not your problem and also being clear that you are not willing to be her solution. It isn't a negotiation. She doesn't get to decide if your reasons for refusing are good enough - don't give any reasons, especially since the biggest reason is your MH, which is deeply personal. Be enigmatic.