I'm so sorry, OP. We had to put a dog down several years ago, and holding his paw while the drugs kicked in was one of the saddest and hardest things I have ever done.
When the time comes, know that there are vets who will come to your house to do it, so your dog can pass in the comfort of his home instead of the vet's office, if that is something you are interested in.
And, check out this article (
but not at work, because you will cry) where this family gave their dog an amazing last day. Doing something similar might make you feel better about the decision you have to make.
http://www.huffingtonpost.com/2014/07/14/dog-last-day-robyn-arouty_n_5585074.html