If it does come to this, repeat “I release you with love and my blessings” three times either when it’s done (if you can be there) or as soon as you find out, if it’s done by the veterinary staff when you’re not there.
Your reactions are normal for a time like this, I have had sick family and sick animals and it messes with your head. Don’t worry about it, we understand. 
See if you can take her something with your scent on, and read this, try tapping this number to anything you have in your home which is linked to her, or smells of her:
Other things to try (check these don’t clash with existing workings):
Possibly also this, especially if her lapping number (as in first link) is 5: Could This Occult CIA Code Relieve Grief & Depression?! - and you could also try it on yourself, long enough to get some rest, because being sleep deprived at a time like this will not help you to make great decisions.