How do you say goodbye?

As I consider foster parenting down the line, I struggle so much with the idea of saying goodbye to children I have grown to love, especially if I know they are being returned to less than ideal circumstances. I am trying not to let it deter me from fostering- I have so much love to give and I want to make a difference, even if only for a few days so a child can feel loved. But I'm not sure how I will be able to cope. How do you do it?