My first question is what are you riding him in in his mouth?
If you have a headstrong horse who you know is going to do certain actions no matter what amount of training you put on him then you need to just occasionally up the ante and make it unpleasant a experience for "Rusty" in this case.
Flay me if you want...
Running into trees is very risky for the horse and for the rider as a means of stopping your horse.
Impaling a branch into the horses face or yours is not unheard of, nor is breaking a neck.
For someone like you who rides with I bet a loose rein or soft contact and think you are into bitless with riding on trails...yes, my upping the ante is that horse would have a short curved shank solid mouth bit in his mouth next trail-ride out.
Yes, intentional he will get poll pressure and jaw pressure to change his train of thought and have him take notice of you and your directions.
I'm not going to play games with a broken mouth, direct rein snaffle bit...I want
some heft to my gently given direction of slow down, turn, halt and stand-still when needed, as needed.
If I need to apply force and muscle the animal I surely can!
You have said Rusty is bull-headed about some things I believe in the past...if this is one of those things no amount of your training response systems can work through, then do what you must to keep you both safer.
And yes, a bit is only as strong as the person holding the reins makes it, the horse dictates
with a good rider how much of a hold you take to their mouth and body.
Your horse, with what you describe is downright bolting which is darn scary to ride and impossible to control = dangerous for both your safety.
Yes, you can only work your issue on the trail...doesn't happen elsewhere and elsewhere he is a gem to ride..
very common that what a horse works in/as in one environment doesn't work everyplace...
Mental stimulation of your lazy nature horse on the trail brings out a different mindset, one you now need to make clearly understood he is not going to like the response he brings to himself when he stops listening to you. He gets what he deserves...
Bit him up, but only use that bit with the capabilities of what it can do if you really take a hold as needed...
Don't be afraid to use that bit either to save your bacon!!
Rusty is smart...he figured out he can do as he pleases running on the trail.
Rusty will very quickly learn he can no longer be a bull and ignore you or he pays the penalty. Rusty has your number and is taking advantage...time to even the score and gain back some respect.
... That is my opinion...