I am so sorry that you have to go through this, but I know exactly what you're going through. I had to put my cat Patches to sleep last November. She was fine one minute and then couldn't move her back legs the next minute. When we brought her to the vet within an hour of this happening, her back legs were blue and she was having trouble breathing. We stayed with her while they did it, because I could not leave my baby while this was happening. This was the hardest decision I ever had to make, but I know it was the best for her. To top it off, all this happened 2 weeks after my daughter was born and on Thanksgiving.
Hopefully your kitty will get better. I will be thinking about your and your kitty.