Take her out back and put a .22 or two in the back of her head, right between the ears. Make sure to be shooting down and into the ground.
I don't mean that as horrible and it ain't for everyone and simply may not be practical depending on where you live (or legal? Maryland probably has rules on even this) but,
there is absolute closure taking your pets life, being the one seeing them off instead of some stranger sticking them with a needle in some strange surrounding. And it is humane. Anyone who says it isn't can kiss my ass. As a side benefit, it is cheaper.
You are doing the right thing by letting her go.
You should be ashamed of yourself. Really.