Jenny is so right, Donna. I know it's hard, but you need to let her go for more reasons than one. You said yourself you cannot keep her, and she will get adopted soon, and she will have others to play with which will make up for being in the cage, and she'll live a long and happy life in her new furrever home - all thanks to you for allowing yourself to think of her long term best interests. The longer you keep her, the longer it's going to take to get her adopted, and the more attached you're going to get to her. And on top of all this, like Jenny says, it frees up a spot in your home for a new foster, which I'm sure will be along in no time.

Be strong, give her lots of cuddles before she goes, but know you are doing what's best for her.