Roni it looks as though you've been caught.
The saddle matter is interesting. If she'll let you ride bareback, then you must experiment with saddle and pads to see where she'll tolerate your weight.
Maybe a treeless endurance saddle with thick padding might work.
I'd be feeding her treats so she sees your presence as a source of pleasure and not just pain.
You'll have to get a feeling of whether she can tolerate walk, trot and canter.
I'd do plenty of work in hand - so she gets to know you without having to take your weight.
But do take care for you not to be associated with her pain. If you plan to mount up, make sure she has had her pain killers. Managing her stomach acids will be a separate issue. Read up on 'ulcers' on the web.
The vet can use steroids to delay arthritis - but it only delays the onset.
One day, she might have to be sent on. In the interim don't let her bite you, but don't make it worse by snapping back at her too harshly.
I hope you think she is worth it.