This is kinda funny but only because my fiance's haflinger did the exact same thing and was the same age and situation(with not being ridden/driven) in 4 years. Know that there is hope...lol. she sounds like a female version of his gelding.
we got Doc after we had him saddled up and ridden after not being ridden for 4 years. He did awesome. Extremely responsive, affectionate, knew whoa and go. was extremely overweight. so we bring him home...first 3-4 rides were perfect, then it's "let's rub up against a tree...or go this way...or lets throw our head and shake...or even maybe bow up and just throw a tantrum... it really was not funny. It was irritating and made it an unpleasant ride. And it was mostly due to the fact my fiance was more of a pushover and would "let" him get away with stuff. they both learned from it. Now after 2 months of persistence, about a 100lb wt loss, a lot of going in circles and some small battles we usually only get a few headshakes a ride....we did try several bits, and found the best for him is just a plain ol' snaffle. Now though he sometimes (especially when we first head out) tries to throw his fit, it's not nearly as bad or as long of one, with over half the time...none. Both of them have learned...and it's good. It's amazing the attitude they can pick up...very quickly when given just a LITTLE freedom.