Firstly, huge hug to you. I know this feeling first hand and it's soul destroying. There are a lot of people whose sympathy will lay soley with the horse, but mine is split between you both.
Ultimately, when you reach the point of considering PTS, you're almost there. There are far, far, far worse fates for a horse than being laid to rest around people they know in a familiar setting with a fully belly. It sounds like you've tried and you're tired. It sounds like she isn't a naturally happy horse, possibly pain related, possibly pain memory related. Yes, you could retire her to a field. Yes, you could keep trying. Yes, you could spend a fortune on more training and schooling. But really, it doesn't sound like she is ever going to be a truly happy horse.
I had a very similar situation with a cracking little horse who was so, so talented but his brain was so far gone. He also had a multitude of veterinary issues that I just couldn't keep at bay. When I put one fire out, a new, bigger, hotter one was lit. I gave him 6 months in the field to see if he'd take to retired life and he just didn't - he was 'ok' but he wasn't a happy horse. So I made the decision, I called the vet, I took a couple days with him and then I held him as he left. He was just 13 years old and it broke me, but it was the right decision for him, I dread to think where he or I would be now if I'd tried to carry on.
Vets can sort all of this out for you and you don't have to hold them. If you aren't going to be there, captive bolt is cheaper. However, I personally (despite working for an animal welfare charity and seeing it done 100s of times) couldn't do it with my own, even thought it is arguably faster and kinder. Either way, you don't have to be there and your vets can organise everything start to finish.
This is an impossible decision and I'm really sorry to say that if you choose to say goodbye, it is going to get harder for a while. It is one of the worst decision you'll ever have to make - I personally think a broken leg or incurable colic is easier to handle as the decision is taken from you. But always know you did right by this horse and she knew nothing but an owner that cared, and that is why this is so hard and hurts so much.
Sending love.