I am so very sorry. I know that nothing anybody else can say will ease your pain. This is one of the absolutely hardest things that most of us have ever had to experience and it's always horrible...sometimes worse than others. And when something bad continues to happen over a course of time, it's the worst. I, too, hope that Osiris will be able to rally...but it does sound bleak at this point.
Remember that she can pick up on your emotions so you want to be as strong as possible in her presence. Give yourself permission to let go when you're not around her. Put on the best face you can for the vet visit and just hold her. Let her feel your strength and take comfort in that. She knows you will do the best thing possible for her...she depends on you. If it turns out to be her final visit, tell her that there will be no more pain or fear...it'll be OK (or whatever you always say to reassure her). And don't be embarrassed to cry in front of the vet and staff. You are so not alone with your reactions.
For whatever it may be worth, I feel as if I've truly absorbed some of your pain while thinking of you and typing this...and I'm sending you some strength to help you through this.
Let us ride this out with you. People are scarcer on here over weekends/holidays, but you still have a lot of shoulders to lean on. Most of us have been in your situation at least once if not over and over again. We're definitely here for you. Remember us when you're at the vet. And after.