It is so much harder to sell a horse when you live with them. They can be so much more than the bad rides! I have only sold a few horses - and I can say that 2 of them were horses that I’d had a bad experience with that I knew I would not be able to get past. They were not bad horses by any means, just bad luck if anything. It sounds like you are not there with your mare, OP - which is good, but it does make for an easier decision.
I have a mare that I really like but was just a nightmare for me to ride for much of the time for years. Like, I started a post last winter about how every ride was ending in tears, and a lot of people suggested I sell her. (Fair enough!) I also just really like her, and while she was a nightmare, I never really lost the faith in her. I tried different trainers and different disciplines: it wasn’t like she was just not good for me or at my barn. She just felt like an impossible nut to crack. Very long story short: there was a physical issue that is resolved and I now have a lovely horse. If you really enjoy your mare that much, maybe it is worth seeing if you can work through it - give yourself a timeline and see where things are in a few months. I know it’s hard in winter in particular, but maybe try something like committing to at least 5 days a week for a month and see if the consistency helps. I am truly happy that I did not give up on mine, but the truth is that if anyone had come along this time last winter and offered me a good home for my mare, I would have sent her packing for $1. It was really that terrible. If it were me and the horse went well for others, I would probably put the word out that she is available to the right person. (Could your trainer find someone who does well with her to free lease her for a while and you can reevaluate things in a few months?)
Also: it’s winter. Everything is extra terrible right now. I really really did not want to ride today: it’s gray and miserable (AGAIN). I told myself, just walk for 15 minutes. Then I trotted, and it was pretty good, and I thought, oh well, might as well canter now. And that’s how I get through winter 
ETA: I firmly believe the kindest thing that you can do for a horse is to train them/make them useful. I’m a “what if” type and I want to make sure that if I can’t keep my horses for whatever reason, they will be useful for someone else. A useful horse will always find a home.
Ultimately my mare had a complete ovariectomy. She was very tense and reactive under saddle, and particularly so in the winter, which is likely why most vets did not immediately think it was a repro-related cause. She did not have granulosa tumors and hormone tests did not show anything unusual, so while we will never know for sure what the exact issue was, the basic theory is that she had significant pain from her ovaries/follicles, and her cycles were not controlled by Regumate. She had multiple muscular-skeletal exams, radiographs, Lyme tests, scoped for ulcers… once we had really ruled out everything else, we fairly aggressively purused the hormonal stuff.
and while we don’t compete anymore (only because she hates being in a stall all weekend), she’s still sound and we still school over fences, xc, and do a ton of dressage. She’s actually got a pretty decent piaffe in there too!
