First of all... big, big, big hugs from all of us to you.
Answering to your question (or pathetically trying to)...I don't know if one really lets go. Our heart just gets numb with time I guess. It's been 3 years since we lost Sidney and just today, on our way back home from our Wed night program at church, I was saying to my husband how much I still miss Sidney. My new shepherds are amazing and I love them dearly. I never, ever compare any of my current dogs to the ones we lost. It's not fair to anybody: the fallen hero, us or the new puppers. But even with 3 dogs running around, being awesome GSDs all the time... I still miss my Sidney. And I still feel that guilty pang of "what if?" for putting him down at 12 yo with his severe spondylosis that finally took away his hind body mobility and control.
It is hard to say goodbye but in the other hand you have an amazing history together, great memories and that legacy never goes. Don't be hard on yourself. It's a lifetime of love and it will take time to heal. It never really goes. The skin just gets tougher I suppose.
Again, super hugs from all of us, 2- and 4-legged alike. I'm sure Sidney and Zeus are having a ball together.