It's hard to lose a pet. We spend so much time with them, they really do become a major part of our lives. We cherish their different personalities and all of their little quirks that make them them. Once that is gone, a hole is left that is not easily filled.
I recently had to put my dog down, and I know how hard it is. I feel for your loss, but at the same time, you are doing good by rescuing them. Never let the loss of them discourage you from the good you are accomplishing.