The first dog I took under my responsibility (I won't say he was mine; I don't own dogs, I care for them) came to me at 12 weeks. He was the sole survivor of his litter, as his mother was too old.
We spent the next twelve years together. A cancer took his health and at nearly twelve years old I had to ask help for him to go peacefully and painlessly. He took his last breath in my harms.
I miss that dog every single day.
You did your best. If you miss that pup, that much,, you did your best. Remember the good times you had and be happy. They don't like to see us down.