I don't think you've made a mistake at all, there are some cats that will never be tamed and, as Liz has found, need to be found a suitable environment for their temperament. I took a cat last year who savagely attacked me and OH, we were utterly devastated by what we assumed to be our failing, and returned her to the rescue she had come from. Only now do I know for sure that it was a glitch in the cat's psyche that caused her attack, not my treatment of her, and was not a reflection of my abilities as a rescuer.
Please don't despair. There are homes for feral and semi feral cats out there, talk to your vet who may keep an eye open for a suitable farm/stable environment especially if he has a large animal concession at his practise. Otherwise as others have advised, ask all your local rescues to be put on their lists.
Meanwhile, although I don't rate Feliway (and Felifriend spray is positively dangerous in my experience) I do love Pet Remedy plug ins and sprays, Zylkene powders for de-stressing their food and a drop of rescue remedy in drinking water.
I also like to give over-anxious cats a stinky valerian toy to beat up, pee on and generally get rid of some pent up aggression with.
Try the Purrs shop
they have some really stinky ones!