Been there, done that, have the the guilt-shirt for it. The vet and the specialist told me that surgery for Polly's laryngeal paralysis would greatly improve her quality of life. She was my baby-girl so I agreed. I had no idea when I signed the consent that I was going to put my girl through all the circles of hades and back. When she left for the bridge 10 months later, I couldn't decide if I was more relieved it was finally over or devastated it was finally over. It took a long time to get past the sensation that someone had gouged my lungs out of my chest using a dull spoon and then poured acid into the hole.
However, Polly has been gone 6 years. When I think of her now, it is always with a smile. About the time she stole my snooty-know-it-all relative's corned beef sandwich. Or the way she could snatch a racquetball out of mid-air after flying across the yard. Or scamming Chester out of his dog chow by pretending a cat was outside. Or especially how she would give dh serious grief for sitting in HER chair when she was ready to go nite-nite. One thing that has helped is knowing she is still here. How else can I explain having a greyhound (Sirocco) that acts like a brittany spaniel (Polly)?
Another thing that helped is knowing that grief comes in layers. Each layer had to be worked through in my own way and my own time before I could move on to the next. Until one day, I realized that my memories of her sweet, loving, rotten, stinking and ornery ways had replaced the ones of her gasping for air.
Please give yourself a break. You made the decisions you did out of love. No one can ever do better than that.