Learning to ride a horse in the bridle, or on the bit, is a really important step in the education of the horse and the rider. It is challenging. Prerequisite for riding in the bridle is having an independent seat (check out my blog if you are interested in further info on this salisburyfarms.net). If you have an independent seat, then basically you encourage your horse forward, when he feels the pressure of the bit he learns to find release when he engages his back and hind end and collects up into the bridle. You have to remember that your horse may be in the learning phase of this process. If so, teaching the horse to lunge with side reins or working with an instructor can be extremely helpful in this process. If your horse is already familiar with this process, then common rider errors include not having enough forward momentum or "pulling" with the reins. If you are not sure what the problem is, I recommend taking a video and watching yourself, or consulting with a top notch dressage or reining training in your vicinity. Ask them for an assessment of your horse and an outline of what they would recommend to assist you. Hope this helps!