Can totally understand your feelings. I cried when we left Potion at the vet to be spayed. I was a total wreck.
Have a good cry and get it out. Know that you did NOTHING wrong, in no way you failed Hoku. Life throws curves. Surgery is going to go fine. You've done your research and are confident in the surgeon. Handle each day, one day at a time. You can do it because of your love for her.
Hugs