know exactly how yr feeling bud, had to do make the same decision with my dog last feb.
only difference was that he wasn't in any pain and still ascting like a puppy even though he was 13years old, but vets had told us he had a tumor and it could cause him a lot of pain at any moment. it's not fair to let them suffer so i take my het off to you for making a very tough decision.

R.I.P