We have had a feral cat since he was a kitten. He was born in our wood pile. He and his brother are the only two we ever saw. Their mom brought them onto our deck to live in a cat house I built them. Eight years later the mom has passed away and one of the boys is completely domesticated and the other lives in our house, sits with us but won't let us touch him. I don't think the Op's cat is unhappy, just not ready to be domesticated. I think the worst thing you can do is take it back to the shelter. Just more trauma.