I think we all can understand those feelings of helplessness, watching our precious Greys decline, and knowing the our time together is more and more finite. But something that I try to hold onto is this: they live in the moment, and don't see the sadness to come. As difficult as it is for us to not see that too, I think we have to try, for their sake. Don't let the monster steal your precious time away from you now because then it will have won twice. Sherman knows he is safe and loved NOW. You are there for him, keeping him safe and as free from pain as possible because you love him so much, and when it is time, and you know you can't keep him from pain any longer, you will still do what you have to do, out of love.
Our thoughts and prayers are with you and Sherman.