I know how that is. I've felt that way myself many times. Because, as hard as it is, there are the ones that you just can't save. I had one of my foster dogs that ended up with late-stage cancer when he was with me. He had been an abuse case and I had had him for two weeks when we found out. So, I kept him for the next six months, until he passed away. The rescue (the old one, not the current one) was going to put him down, but I begged them to let me keep him. I paid for all his medical treatment. It still kills me to think of it, but the only thing that keeps me sane is knowing that maybe his first six years weren't very good. But his last six months were. And maybe, just maybe, that made up for it.
-Rosefern