When is it selfish?

In the last six weeks, Spirit has regressed significantly.  Bringing in additional fosters has limited the time that I spend working with her and resulted in a backwards slid of her behaviors.  While she is still never aggressive, she also prefers to stay as far away from humans as possible… unless there is a treat involved.  If a treat is involved, she has no problem being near me.  She still adores Fred and follows him EVERYWHERE.  In fact, while Fred was recently on bedrest for a torn ligament in his knee, she spent her time doing everything she could to be near him, even when that meant being around people instead of the yard.  However, after three months, there has been no bonding, no connection to humans that I had hoped to see.  That makes me worry and explore all of the questions that I try to avoid because they make my head/heart hurt.  
 
When is it selfish to hold onto a dog?  When is it time to let him/her go? 
 
Is it selfish to give the dog a safe and happy place to live, even if they are taking up resources that an adoptable dog could be using?  I cannot adopt Spirit.  There may be a home out there for her somewhere, but will that home ever find her?  In the meantime, is it wrong to let an anxious dog continue in her discomfort?  Or, is it selfish to give up on that dog, just because they don’t fit our typical mold of what a dog should be?  Is it selfish to take away that hope of her ever finding that perfect family?  Millions of highly adoptable dogs are put down every day, is it wrong to expend all of these resources on one that isn’t?
 
When an animal becomes a danger to other animals or humans, this decision seems to be more clear – eliminate the danger.  However, Spirit is not dangerous in the least.  In fact, she is extremely submissive and gentle. 
 
Fortunately, it appears that my foster load is going to be significantly less in the near future.  With that in mind, my hope is to work with her more intensely and hope for the best.  I wish this was the first time that these questions have come along.  Sadly, they seem to come along far too often and never come with a clear answer.