So many people with more experience than me have been commenting here, but just wanted to say this particular nugget about this mare stands out and really does make me think that it’s her. Because maybe my personal sample size is too small, but this repeated testing of the electric fence as if she doesn’t even remember the cause/effect just seems out of step with typical learning skills and follow-up behavior.
At the risk of sounding like I think horses and dogs are the same and making a massive over-generalization, I will say that we have a saying in our family that dogs’ sweetness and smartness are inversely related (ie, our sweetest dogs have been our dumbest, and our not-so-nice ones… super clever).
We have a 13-month-old incredibly sweet female Great Dane, and while granted, this breed is never going to make it onto a Top 10 Smartest Dog Breeds list, we’ve been honestly shocked at how mentally slow she is. It’s not just having to correct her over and over for the same things every single day-- it’s the baffled look she gives you as you say her name with the command, as if she’s never heard the command in her life.
Anyway. It sounds like you’ve put in a lot of time and effort with her already, not just to train her but to try to understand and help her grow. Whatever happens, you’ll do right by her.