I'm so sorry to hear about your loss. For me, losing a beloved pet is almost unendurable. And making that final decision is so incredibly hard, especially since my pets are much more than pets to me.
My vet of many, many years is wonderful about helping make that decision. She believes, of course, in making all reasonable efforts to save the pet; but if reasonable efforts have failed, we have a realistic talk about (1) whether anything could reasonably be expected to save the pet, and the cost, both in $$ and in suffering for the pet; (2) what quality of life the animal has now, and where it's going (i.e., the expected progression of whatever it is and can palliative measures be reasonably used to alleviate pain and other symptoms) and and would I want the pet to live with increased pain and loss of dignity); and (3) whether my reluctance to let the pet go is really in the best interest of the pet or would I be doing it because I can't face letting the pet go.
I've also found that, in the case of the dogs I've had, the dog has somehow let me know that it is time.
When I lost my beloved Harold and Emily last year within months of each other, I went to the Humane Society and found another doggie within a very few days, because I could not bear the silence and lack of companionship. I found my wonderful Bonnie, a homeless AmStaff terrier, waiting for me there. She is an older girl (about 8 and a half or 9) who hadn't been treated very well in her previous life and had been dumped. She is the sweetest thing, and is now living out her life sleeping upside down on the couch, being cuddled and fussed over, doing fun dog things and giving me lots of sloppy doggie kisses, and I have a loving companion.
One of my friends commented that it seemed awfully soon, but I've always felt that the best way to honor the pet that has passed on is to save another life and give a home and love to another homeless pet. The shelters are full of slightly older dogs who are passed over by younger families wanting a puppy, but many of those older dogs are perfect -- they are calmer, usually housetrained, and they are SOO grateful to find a home and have a second chance at life.
Many shelters will let you take the dog home on a "temporary foster" basis to see how you click before making the final adoption decision. This is what I did with Bonnie, and I knew right away she was the one for me.
Again, I am so sorry for your loss, but I believe you did the right thing.