Well, maybe if the surgery comes out okay, things will improve for the both of you. We can always hope.
So, your therapist hasn't offered "dating suggestions." Is he/she just helping you to work through the depression? What has he/she tried with you so far? if I may ask.
I know you're probably tired of trying more dating sites, but I still encourage you to give PolyMatchMaker a whirl. It's not nearly as active and heavily populated as OKCupid, but it's a pretty good site and sometimes it's the "next rock you turn over" that's hiding the treasure.
Wow, arthritis sucks to have, but most people don't get it at least til they're older. Your sweetie's had some bad bad luck I believe.
But do you feel like she's rejected you on some emotional level -- even if your brain can reason that it's not that, it's the sickness and the pain? Observing the tone of your posts makes me think you might feel emotionally rejected by her ... and thence even subconsciously resentful? I should know, I "blamed my wife" somewhere in the back of my mind when she came down with Alzheimer's. It's not easy being a caregiver, especially if on some level you already feel rejected.
It's times like these when I wish doctors knew how to cure virtually anything. Unfortunately that such knowledge probably lies far in the future.
Perhaps talking things out (or venting a bit) on the forum here helps? in some small way.
I hope you and your sweetie can both get a little relief soon. I have to sympathize.
Please keep us posted on how things are going and let us know if we can help.