I have complete sympathy for the position your in. I was having similar problems with my GSP bitch a couple of months ago, I’d taken her to puppy classes etc, but you’d take her out and she just wanted to please herself, tried all the traditional methods like long check cord all to no avail.
I spent a fair amount of time training before I even let her off loose in an enclosed field, thinking that come the time she’s let off and with all the training I’ve done she’ll be obedient, well it didn’t work for me… Plus the fact if they're on the lead they’re not getting rid of the