
I have personally found that a harness, used on it's own is very poor from a controll POV, but is great for
added controll
(I do have a different breed than you, I have an English Mastiff)
.
You could try using a collar AND a harness at the same time, both with seperate leads. It will give you more confidance knowing you have extra control, it also gives you 2 points of controll, making it easier to handle sudden physical outbursts! Being more confidant will allow you to approach training with a better state of mind and therefore your training will be more effective.
I found this combo to be of great help when my dog was younger, he'd get very excited when meeting other dogs, I had no controll with a collar or a harness when used on thier own and worried about the security of them, used together I could concentrate on just training him, rather than just keeping hold of him and staying on my feet!