I train herding dogs for a living and many for real work on ranches,farms, and operations in addition to trialing or hobby herding which is somewhat different in many cases.
Unlike any other discipline in the dog world herding is a 3 diminsional venue that requires much more adjustment,feedback,reaction dynamics,reading the slightest movement of the livestock,flock dynamics,pressure zone dynamics, draws,pace dynamics,flight and fight zone response,teamwork,trust and respect. Things are changing every second and most often nothing is ever the same from one minute to the next. Good herding requires that the dog not only work with the handler but also make decisions and adjustments on it's own that many times the handler cannot see at the moment. The dog must be given the trust to call many of the shots and the handler must learn when and where to give and take. It also requires learning HOW TO READ LIVESTOCK of various types so that the human understands the pressures working on the dog in order to properly train the dog. The mechanics use in training good herding dogs are sometime opposite of those used in agility,tracking,flyball,SAR,therapy,service dog,protection ect work. This is what makes herding so different and so much more complicated..especially if this is not understood.
I SELDOM deal with dogs who are food or object motivated and from reading all the posts about suggestions I can tell you that these people (doing thier best to help you with your delima) do not know what it is like in this realm as they have to rely mostly on there experiences with reward/target based motivators which only goes so far and often not enough in training certain herding dogs. I know you cannot afford it and probably would not consider it anyway but people like myself and others in the profession often take board and train dogs and then upon pickup provide several hours of herding instruction to the owner. I am not sure how much you are focused in creating a successful herding dog but believe me you just need to find a trainer in your area that does this..takes dogs like yours and turns them on.
Sometimes it is as much about the relationship being a problem as much as needing a special training method none of which I have seen touched upon as of yet in the discussions I read.
Yes in some cases there are failures but honestly I have seen many GSD's with relationship/training problems that turn on when in the rights hands and your situation is not that uncommon. I would suggest joining a AHBA (all breed herding association) herding list and tell your story and you will get help from people who are experienced in this realm and have experienced issues such as yours and have worked thru them....rather than giving up.
I know of several well known herding trainers in your area that could change things for you. From reading all of the suggestions that people are offering in attempts to help you they are mostly all in the wrong direction...And from experience I know why. I use to fail miserable with many of the approaches that people seem to be limited to in other training disciplines..
Good luck and I know you will find a solution if you just research the right sources for answers to your issues.