I have a TB/quarter mix filly that did the same thing. When this happens, she has started using the reactive side of her brain. The best “quick fix” for this is to start off cantering in a field then immediately go into medium/large circles. Do one or two circles to the left, let her straighten out for a few strides, then do one or two circles to the right. Continue this pattern and slowly let her canter straight for more and more strides. If at any point she wants to buck or overreact, immediately take her into circles and change the direction often to get her focused on you again. If she’s too out of control, use a one reign stop for your safety, then start working her in tight circles. Slowing expand the circles as she relaxes. This exercise will slow down her mind and get her focused on you and not bolting. Hope this helped.