Hi Tricia,
Sorry to read your predicament. Iriebabel has given excellent advice.
I too have bad knees although not to the same extent...thankfully. I have no cartilage in my right knee (from a teenage injury) which means bone-on-bone when walking. My left knee has its own cartilage issues & has taken a pounding due to compensating for my R knee. I did 'get away with it' for a few decades until age started catching up. Walking more than 3-4kms was excruciating; I'd be in tears & then pain for hours afterwards. Downhill was even worse.
After a day walk in Samaria Gorge on Crete in 2010...I muttered my most famous, famous last words...'my walking days are over'. Then I discovered the Camino & planned to walk it in 2012. But how was I going to do it?
I spent countless hours at Cradle Mountain National Park (here in Tassie) trying all manner of different things & combinations incl bandages, braces, medications, poles, etc. It usually ended in tears. Eventually I found my Golden Ticket & haven't looked back.
I re-taught myself to walk. It took a lonnnnggg time for it to become 2nd nature but now I don't even have to think about it.
The two main components for me are:
1/ Never, ever straighten my leg; always keep it bent to some degree
2/ (hard to explain) Control each step using the thigh muscle, ie rather than plonking your foot down with each step, ease it down using the strength in your upper legs. This makes you 'step lightly' & takes the strain & pressure off your knees.
In addition, age & ageing is a contributing factor & so is weight (both on your back & on your frame), both of which need to be taken into consideration.
As stated earlier, obviously my knee issues are nowhere near the severity of yours but experimenting with the actual way you walk may offer in part, some solution or relief.
Since finding & adopting my new method, I've walked thousands of kilometres in varied terrain, pain free; I no longer carry medication & even stopped packing my trusty knee bandages.
I'm so thankful I found the answer for me...I can't imagine not having found the thing I love to do most (long distance walking) or had the experiences I've had since I ignored my own words back in 2010.
I congratulate you on not giving up...hopefully you will find a way to manage the issue so you can, not only continue to walk, but also enjoy it without pain being the overriding factor.
Good luck, I wish you all the very best.