My heart goes out to you . . . I know how incredibly hard it is. It does sound like you've gone above & beyond with your dog. Only you can make the decision; but it sounds like your dog is ready. I know what you mean with "not being able to give them 100%". Mine were all my original babies until I had a son 2 years ago. I feel so guilty for not being able to play with them and cuddle with them as much as I used to (although they are still spoiled rotten). I think you know in your heart that your dog is ready; sounds like he's had a wonderful wonderful life and he has been truly blessed to have you for a mom (the world needs more people like you).
When you are ready to bring the dog in, the vet should put a catheter, then bring him to a room and give you some time alone with him. Then they will come back in with the meds, but will allow you to stay in the room and pet him, kiss him, & talk to him while they adminster the meds. It's harder on you being present, but better for the dog that way. Yours is the last voice he hears and he feels loved and safe and secure and not scared.
The important thing about having the catheter is so they can get the dosage in in the proper amount, without accidentally missing the exact place where they should inject it, and without stressing your dog. I have had SEVERAL people tell me that they didn't put the catheter in, and it took several injections, which is even more heartbreaking for you, watching your dog get multiple shots when he should be peacefully going to sleep.
I had to put down my 2 cats, 3 months apart, when they were 16, and another cat in May when he was attacked by a dog. I have 3 dogs - my pitbull will be 11 in November and she is the SWEETEST - I love pitbulls. They get a lot of bad press from people who don't know anything about them. Mine had surgery when she was 7 for her leg (when everyone else said, just put her down and get another dog). We also have 2 more big dogs. I dread the day I have to put one of them down, but I know we will all be reunited on the Rainbow Bridge someday.
Good luck to you Kristy. I will be thinking of you and your dog. If you would like to, feel free to e-mail me at ____@____.com