When riding my horse, he has started tossing his head when I'm trying to turn or stop. What could be

Head tossing is one of many possible behavioural responses to pain in the mouth when being ridden or bitted. Other behavioural problems can include: tilting the head, chewing the bit, opening the mouth, drooling of froth from the mouth, pulling hard, working behind the bit, lugging on one rein, not collecting well, rearing and even bucking, not wanting to have the bit inserted into the mouth etc The pain could be coming from any part of the mouth, so a full oral examination by a skilled and qualified practitioner is indicated. The most important part of a performance float, in terms of bit comfort, is contouring of the first cheek teeth (often referred to as a "bit seat"). To do a good "bit seat" the wolf tooth needs to have been removed first. It must be stressed that the bit seat needs to be done so that it lasts long enough time span (at least 6-12 months), but not so severe that it creates new problems. So again, a qualified, insured, well known and respected, local practitioner is the best option when it comes to having safe, reliable and trustworthy work done on your horse's mouth.