Howard,
I feel your pain knowing what is in front of you. We had the same situation last Christmas when we lost our beloved Sasha. The ride there was the hardest part, but then laying her down on that table, I felt like I didn't do enough to save her. But we did....and so did you guys. These pets light up our lives and ask nothing in return other that our affection...that's why it's so hard to do.
The pain is hard, but it did not pass quickly. We still think about her all the time as we have one of her sons from her only litter.
Light up your cigars and feel the support from your friends, but in the end it's just you and your wife left to grieve. Just give it time, you will never forget, but eventually it won't hurt so much. The constant reminders after his passing will help you over his lose. Where he used to sleep, when he would bark and you would run, his funny little habits that made him special (and a pain in the ass!) will put a smile on your faces when you remember....that's when the healing starts.
You gave him a wonderful life and if you could ask him, I'm sure he would agree.
Hang in there, I'll be thinking of you guys tomorrow when I put up a toast to your great friend.