I'm on my first foster experience as well. It will be very difficult to give this dog up. She is such a sweetie. However, she doesn't fit the bill for what I want in an ACD and even though DH is head over heels for her, he also realizes that in the long term, she isn't the dog he wants.
I would be ECSTATIC to see her placed in a sport home, doing either agility or flyball. She's so drivey, I think she would be awesome at either. Maybe even disk with the right person.
So, yes, I will be sad to see her go... but she isn't right for our home and thus stays a foster.
Only you can decide what's best for you and your home. Imagine yourself a few years down the road. Do you think that you will be happy with a dog that is opposite of what you originally wanted out of a dog? There will always be more dogs in rescue. This dog could find a perfect home where they will love it forever. The right dog is out there for you (whether it be rep. bred or rescued). You just have to decide whether that right dog is the one that is cuddled up next to you now, or one you haven't met yet.