I am so sorry you are dealing with this much frustration. I am an avid pet lover. I love my cat she is my baby. That said if my cat caused me such distress, and interfered with my well being by consistently peeing/marking her territory. I would get rid of her. Do you have such an emotional tie to her that you just cannot live with out her? In my experience cats that age once they start the behavior it is so hard to break them of it. I wouldnt be telling this to someone who has the ability to possibly fix it. I just know this is leaving you very fragile and you have carried this load long enough. You dont deserve to feel guilty because of it.

You have to give yourself the permission to do the right thing for YOU. You can allow someone to take this burden from you so you don't suffer. People will be die hard animal lovers saying how dare you give up on your cat or how dare you euthanize the cat. This is your mental well being. They are not the ones suffering like you are. You make your decision. Give yourself the permission to do what you really want to do.

I wish you well and I hope you find less stress