bit between one’s teeth

To face up resolutely to a hard task: “Ralph is having a difficult time in medical school now, but once he takes the bit between his teeth, there’s no stopping him.” The bit is the part of a bridle that fits into a gap between the teeth in the back of a horse’s mouth. By pulling on the reins, which in turn pull the bit back against the horse’s mouth, the rider is able to stop or turn the horse. If the horse takes the bit between its teeth, it can’t be stopped.