This is such a difficult place to be - watching a loved one go through sickness. It's the most helpless and devastating feeling and I've been there too many times myself. I still tear up when I think about my little lady Sphinx being at the vet getting tested, not being able to stay with her because of the pandemic. Coming home and laying in the fetal position waiting, dreading for the phone to ring but eager for it to ring so I can get some answers. Luckily it all happened quick for me this time rather than having it drag over several months. But I couldn't tell you which was more painful. With Sphinx, it came down to a blood transfusion too - but the vet warned me that it might not help at all. I couldn't put her through that. I couldn't let her suffer anymore.
I'm sorry you are stuck in that hell now. You have some very difficult choices to make. It's unfair that when we make the choice to let them go so they don't suffer, we are then the ones that suffer... But at least we don't have to suffer alone. This site has amazing people who understand what most of the world does not - the grief of losing a family member, even if they have fur, feathers or scales.
“Until one has loved an animal, a part of ones soul remains unawakened” ~ Anatole France