One thing to think about: If she's cranky, she may have good reason. She has arthritis in her hocks. It probably hurts. Depending on what type of arthritis she has, exercise is going to help "keep her moving" and keep her feeling better, but we've got to keep her feeling as good as possible to minimize discomfort, pain, and resentment.
This is exactly why I put my gelding down last year at the young age of 14. I was doing everything I could for him medically to keep him comfortable, but I couldn't ride him anymore and he was just turning into a cranky and aggressive horse ..... that wasn't him. He was hurting. (Not suggesting you put your mare down or anything like that. Just want to point out she may be hurting and that's why she doesn't have the best attitude.)
I know money is nice to have but if you think about all the expensive injections and treatments she's going to need to stay sound and stay happy, you may have better luck with giving her away to a good home.
She just doesn't have the attitude I, as an individual like. She doesn't want to try for plain ring work, only things she wants to do. I like a horse that tries at anything I point him to. I can see her loving a trail home or to a young kid who wants to do riding games/just have fun