Not dogs, but I foster ferrets. Though, I also run the rescue, so my situation is a bit different.
I've been on both sides. There has been a ferret I was glad to see go to their forever home, a couple I hate that I had to give to their new home, and a couple I have kept.
Both Nacho and Ella are foster failures. Ella is a special case, as I *was* keeping an eye open for a young female ferret to add, after I lost Rascal. She just happened to be the right ferret, probably not the best time. I was fostering her, somebody showed interest in her, and I had to make the quick decision to adopt before they could. That said, absolutely no regrets for either of them.
Wedgey, I wish I would have kept. I wish I could have kept. Unfortunately, she was very obviously not happy with other ferrets, so it was not in her best interest to keep her.
Gucci (Spock), who was bonded with Prada, passed away while in my care. I wish he hadn't, because I would have kept him in a heartbeat. He was just so, so sweet. That said, his cage mate, Prada, was complete opposite, and I was happy for him when he got a new home. He just didn't fit in here. Same as Marlee, sweet ferret, but she was ferret aggressive, so it was nice and relieved much tension when she moved on.
I think it's normal to, especially with your first foster, want to keep them. You really do have a bond with them, because they depend on you. Only you could know though, if you really do want to keep her. There's definitely a special feeling you get when you hand them off to their new home, especially those that are with you for a while.