I just responded to a blog and recommended the poster let go of that they cannot control. As I was about to hit save, I was handed a letter to the editor from a coworker and it just said this:

John Wesley said,
"Do all the good you can,
By all the means you can,
In all the ways you can,
In all the places you can,
At all the times you can,
To all the people you can,
As long as ever you can.”

I thought it was completely relevant. That's all any of us can ever do - our best. Some of us are in impossible situations and the best we can do is step back and realize this is a situation, or a time or a person we cannot help. By not letting go, we aren't doing our best with the people and situations we can "do all the good" we can in.