Your mechanics might also be part of the problem. Your hands may not be steady enough, or since you mentioned loosening reins? It could be you are heavy handed and this horse resents it more than most.
But if you are having trouble after a couple of years and it hasn't stopped, then I too think you need a trainer, for you as well as horse. Just because you have ridden and worked with some green horses, does not mean you really know what you are doing. Hence the trouble you are having with this horse.
The head tossing could be a sign of bit problems, heavy hands, reins too tight, curb strap too tight, pain issues, rider unsteadiness or a myriad assortment of many other things.
But whatever is going on, you are not winning this battle. No horse should continually test each time it is ridden.
You need to figure out what is going on, and learn how to correct it. And that means a trainer for you and the horse too.
Horses make me a better person.