My heart goes out to you Witness 007. I know only too well what that pain is like. When our dog died we were all overwhelmed with grief and still miss him terribly, but thank goodness the heart wrenching pain has now eased off.
Also, please rest assured that you are quite normal in your feelings of guilt, wondering if you could have done more at the time. I was exactly the same and tortured myself for ages as I went over and over his last days in my head thinking of ways I could have made things better. This is all part of the normal grieving process and WILL pass in time even though you may not believe it right now.
Hopefully by reading these posts it will help knowing you are not on your own with your unbearable pain and anyone who has lost a beloved pet will know the pain and the emotions you are feeling right now. Try not to beat yourself up over it. Your lab was really lucky to have had such a happy life with owners who adored him.
Take care Witness 007. Please be assured time is a great healer