If your horse is pulling th the contact and you are finding you are bulling as well, then the contact was established backwards, from hand to leg rather than leg to hand. You need to push forward with your legs and form a barrier with your hands that the horse cannot pass, pushing from the inside leg to outside rein. Any pressure in your hands should have been put there by your leg, and not be a result of you taking a hold of their mouth. Anything that you take with your hand you have to give back. Once you get your horse moving off your inside into the outside rein, they will soften into the contact.
I personally establish contact from back to front. In other words I want my horse round and moving forward into my hands. After your horse has developed a constistant forward movement they will learn to accept contact