It sounds like you're doing everything you can. Its a really wonderful thing to help someone in crisis, especially over a beloved pet but you can't give away everything or you would no longer be able to help anyone. My sister had a cat who developed polyps in her throat and ears, after about $1,000 in vet bills they figured out that a $5,000 surgery was the only thing that could save her life. My sister is a student and although fully capable of caring for Sukis regular bills such an expensive surgery was beyond her means. The vet helped as much as she could with Suki's bills but in the end couldn't swallow the cost herself (understandably). The vet instead organized for the humane society to take ownership of Suki and for her to get the surgery there. In the end the humane society decided to euthanize Suki instead of paying for the surgery. We were devastated and furious with the HS since the only reason we gave her up was to save her life but we are still very grateful to our vet for going above and beyond in an attempt to help her. It doesnt always work out but the effort's not lost and even if the owners are ungrateful its all about the welfare of the animal anyways.
I've digressed a little but the gist is do what you can and try not to blame yourself for what you can't, there are factors here beyond your control.
I've digressed a little but the gist is do what you can and try not to blame yourself for what you can't, there are factors here beyond your control.