Doodledog
Can you tell her that you're not feeling up to hosting just now, and that you'll see how you feel next year? You don't need to go into detail - just leave it at that.
If she comes back asking again next year, you can rinse and repeat until she gets the message.
Don't offer fake offer excuses and half promises you don't intend to keep. Just tell her the truth,
Send her a pretty floral card , there is just enough room inside a card for a brief message.
" Dear Betty,
This is a difficult thing to write to old friends but in the last year or so, as age creeps up I have found it increasingly tiring to host guests in my home. So I've decided no more invitations to stay. Sorry about that.
Wishing you all the best