I haven't had time to trawl through all the posts, and I can see the problem from both sides. But the question for the OP has to be "What would you do if you didn't have that exit route?". In other words, what if the horse was yours? How would you put the horse first if you couldn't afford the vet bills? PTS? Suck it up and do the best for the horse? Or try and get a charity to deal with the problem? Just asking.
And why would that make any difference anyway? I've seen threads on here from people who can no longer claim for treatment on their insurance and they're worried about affording it on their own. They've often been told they've done the best for the horse etc and could have it PTS with a clean conscience. They haven't been lambasted for saying they can't afford the bills any more, and that's been when they've owned the horse ie had full responsibility.
Regardless of whether you think OP is morally right or not to return the horse, the issue is that the owner is refusing contact whilst the OP is paying for vets bills for a horse she doesn't want. That's hardly showing a lack of responsibility. It's quite obvious to me who is the irresponsible one here, and it's not OP.
OP is thinking of herself, you're right. She's also thinking of her own horse, and also the loan horse whose vet bills she is footing even though she plans on returning it to the owner.
Let's put this the other way- would you put your horse out on loan and then ignore all contact from the loanee? Is that what a responsible owner would do? Know that ssomeone can't afford the vets bills for it and may actually run out of money and be physically unable to pay the vets bills as much as they'd like the vet out?
Evidently some of you have so much money coming out of your ears that you never have to worry about chosing between paying the vets bills for a horse you no longer want (perfectly reasonable, the whole point to a loan) and being able to eat.
Also wonder (in my long and irritated rant, almost done, sorry!) What response OP would be getting if her reason for terminating the loan was because she was scared of the horse. I had a loan once that I was terrified of, she would pin me up against the gate. We didn't have a contract yet drawn up, and the owners had just booked an Xmas break so were keen for us to keep her. We told them if they didn't take her back we'd just drop her off at the place we got her from anyway. somehow I doubt she'd being vilified for being a coward.