I have been working with a morgan mare since she was five (and now she's six :). But recently half-way through our rides, even if we have only been working for ten minutes, she will start getting extremely stubborn. She lay her ears back and stop dead in her tracks. If I try to keep her moving she'll start going backwards in circles and fling her head around and bump my foot. I keep pressure on her, even using a crop when the need arises ,though I hate doing it, and she'll go however she'll throw a buck in here and there. If I have no control and I feel frustrated, I go back to where I we have a working connection even if it is the walk. I only stop our ride when she is finally good, and I try to make them short working our why up to a longer time. However, each time I get back on she keeps on being naughty so I would love so advice.
p.s. And I'm pretty sure that it is not her tack. I use a bitless bridle and a comfy gel-like girth.