I'm no expert on... well anything!... and my cocker is a pet show cocker, not a worker. But I see a lot of similarities with my dog Henry.
I've had him on the long line in fields for at least four months now. He turns straight back to me if I call or whistle, can turn away from other dogs (a big distraction) and even birds or squirrels if I spot them a second before he does. Playing fetch with a ball - I throw it a short distance and he reliably brings it back. But... OH took him out this weekend and twice he threw the ball much further than I normally do, close to the boundary of the area we normally work in. Henry hared after the ball, then went straight past and kept running. It's like once he hits a certain speed he can't stop.
I'm working on a stop, but he really struggles with it. I reinforce the whistle half-a-dozen times every walk, but sometimes when his ears are turned off, he'll ignore it. He stays on the lead for all other walks, and trots reasonably close to me on a loose lead, sniffing and snuffling. But if I unclipped that lead, he'd be off like a rocket.
I won't give up on the training, but there is a part of me that wonders if I'll ever be able to walk him off lead. Letting him out of my sight for even a short time is just not an option in suburban London. Other dogs and roads are a significant danger. So for now I am still incorporating the training into every walk, but I've decided to not focus on what may be an unattainable goal, because it's making me miserable. We have great walks with Henry either on the long line or attached to my belt on a 2 metre line. If that turns out in the long run that he can never go off lead, I have to accept it's not the end of the world.