My friend has a horse that is very much like your mare. In his case, he was overfaced/brain-fried at a young age and decided he did. not. want. to. play. anymore. He’s been through countless trainers but he has been this way for over 20 years so it’s not going to change: some days, he is happy to jump and will march right around and be champion. Other days he will spin and dump you at the in-gate. He has some venues he hates, others he loves. One small distraction will shut him down and you won’t get him back. He does not easily forgive mistakes and will hold a grudge. However, he is perfect on the ground and on the flat, and is usually pretty happy to jump at home as long as it’s within his comfort zone and he doesn’t get pushed or drilled. Basically, jumping has to be his idea so you go with it when he’s in the mood, and don’t bother trying when he’s not.
When he was younger and my friend was still showing him, she had to be happy with whatever horse stepped out of the trailer that day. If he didn’t want to play, she’d just scratch and would pull her other horse out instead. If this horse had gone to anyone else, he would have been passed around 10 times by now and probably would have ended up in a bad situation (or worse), but lucky for him my friend doesn’t sell horses and she was able to buy a second (much more reliable) horse.
So, if you’ve ruled out pain/physical issues, and it seems your mare simply just doesn’t want to jump, I think you need to listen to your horse and let her do what she wants to do. Pushing her isn’t going to help, it’s only going to end in injury or heartbreak. If she wants to do dressage, great. If she wants to be a broodmare, great. Sometimes horses are who they are and they’re not going to change and you have to either accept their limitations/quirks and work within those parameters, or decided to move on and find her a situation where she will be happy.
Best of luck.